docs: add developer documentation under /docs and update CONTRIBUTING.md

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Co-authored-by: petesramek <2333452+petesramek@users.noreply.github.com>

Author: Copilot

CONTRIBUTING.md

@@ -2,6 +2,20 @@
 
 Thank you for your interest in improving this library!
 
+## Developer Documentation
+
+In-depth developer guides are in the [`/docs`](./docs/README.md) folder:
+
+- [Local Development](./docs/local-development.md) — build, test, and format commands
+- [Testing](./docs/testing.md) — how to write unit tests
+- [Benchmarks](./docs/benchmarks.md) — how to write and run benchmarks
+- [Composite Actions](./docs/composite-actions.md) — reusable CI actions catalogue
+- [Workflows](./docs/workflows.md) — CI/CD pipeline overview
+- [Branch Strategy](./docs/branch-strategy.md) — branch lifecycle and environments
+- [Versioning](./docs/versioning.md) — branch naming and the version pipeline
+- [API Documentation](./docs/api-documentation.md) — DocFX and the API reference site
+- [Extensibility](./docs/extensibility.md) — how to add new encoding algorithms
+
 ## Guidelines
 
 - **Follow code style:** Use `.editorconfig` and run `dotnet format`.

docs/README.md

@@ -0,0 +1,23 @@
+# Developer Documentation
+
+Welcome to the developer documentation for **PolylineAlgorithm**. This section covers everything you need to contribute code, tests, benchmarks, and automation to the project.
+
+## Contents
+
+| Document | Description |
+|---|---|
+| [Local Development](./local-development.md) | Build, test, and format the codebase locally |
+| [Testing](./testing.md) | How to write and run unit tests |
+| [Benchmarks](./benchmarks.md) | How to write and run performance benchmarks |
+| [Composite Actions](./composite-actions.md) | Reusable GitHub Actions used across workflows |
+| [Workflows](./workflows.md) | CI/CD pipelines and how they connect |
+| [Branch Strategy](./branch-strategy.md) | Branch lifecycle and environment mapping |
+| [Versioning](./versioning.md) | Branch naming convention and the version pipeline |
+| [API Documentation](./api-documentation.md) | How DocFX generates and publishes the API reference site |
+| [Extensibility](./extensibility.md) | How to add new encoding algorithms |
+
+## Quick Links
+
+- [Contributing Guidelines](../CONTRIBUTING.md)
+- [API Reference Site](https://petesramek.github.io/polyline-algorithm-csharp/)
+- [GitHub Issues](https://github.com/petesramek/polyline-algorithm-csharp/issues)

docs/api-documentation.md

@@ -0,0 +1,136 @@
+# API Documentation
+
+This document explains how API documentation is generated and published for PolylineAlgorithm.
+
+## Toolchain
+
+Documentation is built with [DocFX](https://dotnet.github.io/docfx/), a static site generator for .NET API references and Markdown guides.
+
+## Repository Layout
+
+```
+api-reference/
+├── api-reference.json        # DocFX build manifest (site generation)
+├── assembly-metadata.json    # DocFX metadata manifest (API extraction only)
+├── toc.yml                   # Top-level table of contents
+├── index.md                  # Landing page
+├── favicon.ico
+├── media/                    # Images and other static assets
+├── guide/                    # Markdown guide articles
+│   ├── toc.yml
+│   ├── introduction.md
+│   ├── getting-started.md
+│   ├── configuration.md
+│   ├── advanced-scenarios.md
+│   ├── sample.md
+│   └── faq.md
+├── 0.0/                      # Auto-generated API metadata for version 0.0
+├── 1.0/                      # Auto-generated API metadata for version 1.0
+│   └── *.yml                 # DocFX apiPage YAML files (one per type)
+└── _docs/                    # Build output (excluded from DocFX content, gitignored)
+```
+
+## Two DocFX Manifests
+
+### `assembly-metadata.json` — API Extraction
+
+Used by the `documentation/docfx-metadata` composite action during CI builds to extract XML documentation comments from source code and produce DocFX-compatible YAML files.
+
+```json
+{
+  "metadata": [{
+    "src": [{ "src": "../src", "files": ["**/*.csproj"] }],
+    "dest": "temp",
+    "outputFormat": "apiPage"
+  }]
+}
+```
+
+- **Input:** All `.csproj` files under `src/`
+- **Output:** YAML files in `api-reference/temp/`, then copied to `api-reference/<version>/`
+- **When it runs:** Automatically after every successful `build` workflow run. The resulting YAML files are committed to the repository under `api-reference/<friendly-version>/` (e.g., `api-reference/1.2/`).
+
+### `api-reference.json` — Site Build
+
+Used by the `documentation/docfx-build` composite action to build the full documentation site.
+
+```json
+{
+  "build": {
+    "content": [
+      { "files": ["index.md", "toc.yml", "guide/*.{md,yml}"], "exclude": ["_docs/**"] },
+      { "dest": "", "files": ["*.yml"], "group": "v1.0", "src": "1.0" },
+      { "dest": "", "files": ["*.yml"], "group": "v1.1", "src": "1.1" }
+    ],
+    "output": "_docs",
+    "template": ["default", "modern"]
+  }
+}
+```
+
+- **Input:** Markdown guide articles + versioned API YAML files
+- **Output:** Static HTML site in `api-reference/_docs/`
+- **When it runs:** During `release.yml` (automatic on every push to `preview/**` or `release/**`) and `publish-documentation.yml` (manual trigger).
+
+## Publishing Flow
+
+```
+src/ changed
+    │
+    ▼
+[build.yml] → docfx metadata → commit YAML to api-reference/<version>/
+                                        │
+                                        ▼
+                              [release.yml] or [publish-documentation.yml]
+                                        │
+                                        ▼
+                              docfx build → api-reference/_docs/
+                                        │
+                                        ▼
+                              GitHub Pages → petesramek.github.io/polyline-algorithm-csharp
+```
+
+## Adding a New Version to the Site
+
+When bumping the version to `X.Y`:
+
+1. The `build` workflow automatically generates metadata YAML files into `api-reference/X.Y/` after the first build on the new branch.
+2. Add a new entry to `api-reference.json` under `build.content`:
+   ```json
+   { "dest": "", "files": ["*.yml"], "group": "vX.Y", "src": "X.Y", "rootTocPath": "~/toc.html" }
+   ```
+3. Add a matching group definition:
+   ```json
+   "vX.Y": { "dest": "X.Y" }
+   ```
+4. Add the new version to `api-reference/toc.yml` so it appears in the navigation dropdown:
+   ```yaml
+   - name: vX.Y
+     href: X.Y/PolylineAlgorithm.html
+   ```
+
+## Writing API Documentation
+
+All public types, interfaces, and members must have XML doc comments. DocFX picks these up automatically:
+
+```csharp
+/// <summary>
+/// Encodes a sequence of coordinates into a polyline string.
+/// </summary>
+/// <param name="coordinates">The coordinates to encode.</param>
+/// <returns>An encoded polyline string.</returns>
+public string Encode(IEnumerable<(double Latitude, double Longitude)> coordinates) { ... }
+```
+
+After merging a change, verify the rendered documentation at the [API Reference Site](https://petesramek.github.io/polyline-algorithm-csharp/).
+
+## Local Preview
+
+To preview the documentation locally:
+
+```bash
+dotnet tool update -g docfx
+docfx build ./api-reference/api-reference.json --serve
+```
+
+Then open `http://localhost:8080` in your browser.

docs/benchmarks.md

@@ -0,0 +1,133 @@
+# Benchmarks
+
+This guide explains the benchmark project structure and how to write and run performance benchmarks.
+
+## Project Structure
+
+All benchmarks live in the `benchmarks/` directory:
+
+```
+benchmarks/
+└── PolylineAlgorithm.Benchmarks/
+    ├── PolylineEncoderBenchmark.cs    # Benchmarks for AbstractPolylineEncoder
+    ├── PolylineDecoderBenchmark.cs    # Benchmarks for PolylineDecoder
+    ├── PolylineEncodingBenchmark.cs   # Benchmarks for PolylineEncoding helpers
+    ├── Program.cs                     # BenchmarkSwitcher entry point
+    └── PolylineAlgorithm.Benchmarks.csproj
+```
+
+The project targets `net8.0`, `net9.0`, and `net10.0` and references the main `PolylineAlgorithm` library along with the `PolylineAlgorithm.Utility` helper project.
+
+## Framework
+
+Benchmarks use [BenchmarkDotNet](https://benchmarkdotnet.org/). Key packages:
+
+| Package | Purpose |
+|---|---|
+| `BenchmarkDotNet` | Core benchmarking framework |
+| `BenchmarkDotNet.Diagnostics.Windows` | Windows-specific diagnostics (ETW) |
+
+## Writing a New Benchmark
+
+1. Create a new `.cs` file in `benchmarks/PolylineAlgorithm.Benchmarks/`.
+2. Add the standard copyright header.
+3. Annotate the class with `[MemoryDiagnoser]` if you want allocation tracking.
+4. Use `[Params]` to parameterize input sizes.
+5. Mark benchmark methods with `[Benchmark]`. Mark one with `[Benchmark(Baseline = true)]` when comparing variants.
+6. Use `[GlobalSetup]` to prepare shared data once per parameter combination.
+
+Example:
+
+```csharp
+//
+// Copyright © Pete Sramek. All rights reserved.
+// Licensed under the MIT License. See LICENSE file in the project root for full license information.
+//
+
+namespace PolylineAlgorithm.Benchmarks;
+
+using BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Engines;
+
+/// <summary>
+/// Benchmarks for <see cref="MyEncoder"/>.
+/// </summary>
+[MemoryDiagnoser]
+public class MyEncoderBenchmark {
+    private readonly Consumer _consumer = new();
+
+    [Params(1, 100, 1_000)]
+    public int CoordinatesCount { get; set; }
+
+    private (double Latitude, double Longitude)[] _data = [];
+
+    [GlobalSetup]
+    public void Setup() {
+        _data = [.. RandomValueProvider.GetCoordinates(CoordinatesCount)];
+    }
+
+    [Benchmark(Baseline = true)]
+    public void EncodeArray() => new MyEncoder().Encode(_data).Consume(_consumer);
+}
+```
+
+## Running Benchmarks Locally
+
+Benchmarks **must** run in Release configuration to produce meaningful results:
+
+```bash
+dotnet run \
+  --project ./benchmarks/PolylineAlgorithm.Benchmarks/PolylineAlgorithm.Benchmarks.csproj \
+  --configuration Release \
+  --framework net10.0 \
+  -- --filter '*'
+```
+
+### Useful CLI flags
+
+| Flag | Description |
+|---|---|
+| `--filter '*'` | Run all benchmarks |
+| `--filter '*Encoder*'` | Run benchmarks whose name contains `Encoder` |
+| `--runtimes net8.0 net9.0 net10.0` | Run on multiple runtimes |
+| `--exporters GitHub` | Export results as GitHub Flavored Markdown |
+| `--memory` | Enable memory diagnoser output |
+| `--iterationTime 100` | Iteration time in milliseconds |
+| `--join` | Merge results from multiple runs |
+| `--artifacts <path>` | Output directory for results |
+
+### Example: multi-runtime run with GitHub export
+
+```bash
+dotnet run \
+  --project ./benchmarks/PolylineAlgorithm.Benchmarks/PolylineAlgorithm.Benchmarks.csproj \
+  --configuration Release \
+  --framework net10.0 \
+  -- --runtimes net8.0 net9.0 net10.0 \
+     --filter '*' \
+     --exporters GitHub \
+     --memory \
+     --iterationTime 100 \
+     --join \
+     --artifacts /tmp/benchmarks
+```
+
+## Benchmarks in CI
+
+The `pull-request` workflow runs benchmarks on Ubuntu, Windows, and macOS when `vars.BENCHMARKDOTNET_RUN_OVERRIDE == 'true'` or when building a release branch. Results are uploaded as artifacts (`benchmark-<os>`) and written to the workflow step summary as a Markdown table.
+
+Relevant workflow variables:
+
+| Variable | Description |
+|---|---|
+| `BENCHMARKDOTNET_WORKING_DIRECTORY` | Working directory for `dotnet run` |
+| `BENCHMARKDOTNET_RUNTIMES` | Space-separated runtimes to bench |
+| `BENCHMARKDOTNET_FILTER` | Filter expression passed to `--filter` |
+| `DEFAULT_BUILD_FRAMEWORK` | Framework used with `--framework` |
+| `BENCHMARKDOTNET_RUN_OVERRIDE` | Set to `true` to force benchmark run on PRs |
+
+## When to Add or Update Benchmarks
+
+- Add a new benchmark file when introducing a new encoding/decoding code path.
+- Update an existing benchmark when changing the algorithmic implementation of an existing path.
+- Attach benchmark results to pull requests that affect performance-sensitive code (see [CONTRIBUTING.md](../CONTRIBUTING.md)).