Add custom build docs

Author: mythz

content/docs/deployment/custom-build.mdx

@@ -0,0 +1,91 @@
+---
+title: Custom llms .py Build
+description: How to create a custom build of llms .py with only the extensions you need
+---
+
+## Lean Core + Extensibility
+
+To reduce bloat, **llms .py** was intentionally built around a lean core with support for extensibility. Most major features are encapsulated in **extension folders** that can be layered on to create a custom build with only the features you need.
+
+### Benefits of This Approach
+
+- **Minimal Footprint** - Deploy only what you need, reducing dependencies and attack surface
+- **Faster Startup** - Fewer modules to load means quicker initialization times
+- **Easier Maintenance** - Isolated extensions are simpler to update, debug, and replace
+- **Flexible Deployment** - Create purpose-built distributions for different use cases (e.g: CLI-only, API server)
+- **Cleaner Codebase** - Separation of concerns keeps the core focused and extensions self-contained
+
+## Creating a Custom Build
+
+The [custom-build](https://github.com/ServiceStack/llms/tree/main/custom-build) directory demonstrates how to create a custom build of the llms .py CLI/Server with a curated set of extensions. This allows you to deploy a lightweight version of the application with only the specific functionality you need.
+
+## Custom Builder Script
+
+Use `./build.sh` to create a custom build of llms .py with only the extensions you want, where running `./build.sh` without any arguments:
+
+```bash
+cd custom-build
+
+./build.sh
+```
+
+Generates a minimal custom build inside a new `llms` directory containing just the [providers extension](/docs/extensions/built-in#providers-extension) and necessary configuration files:
+
+```
+llms/
+├── .llms/            # New ~/.llms home for this build
+├── extensions/       # Built-in Extensions directory
+│   └── providers/    # Built-in providers extension
+├── llms.json         # llms.json config
+├── providers.json    # models.dev providers.json config
+├── main.py           # llms.py implementation
+├── llms.sh           # llms.sh startup script
+```
+
+This minimal encapsulated build doesn't contain any UI or tools, but does contain the [providers extension](/docs/extensions/built-in#providers-extension) with support for all **24 providers** and their **530+ models** in both [CLI mode](/docs/features/cli) and its Open AI Compatible `/v1/chat/completions` endpoint.
+
+### Startup Script
+
+Once built you would use the `llms.sh` startup script for all commands instead of the `llms` command, e.g:
+
+```bash
+./llms.sh "What's the Capital of France?"
+```
+
+### Minimal UI Build
+
+Set the `UI` environment variable to `1` for the minimal UI build which includes the [app](/docs/extensions/built-in#app-extension) and [tools](/docs/extensions/tools) extensions.
+
+```bash
+UI=1 ./build.sh
+```
+
+### Custom Extensions
+
+For a custom build set the `EXTENSIONS` environment variable to a comma separated list of extensions to include in the build. E.g. this includes the same as as the Minimal UI build:
+
+```bash
+EXTENSIONS="providers,app,tools" ./build.sh
+```
+
+> Including any extension that requires a UI will automatically include it in the build.
+
+### Add external extensions
+
+With [full CLI support](/docs/features/cli), your custom build can use the [extension management commands](/docs/extensions) to manage installed extensions, e.g., you can add external extensions to your custom build by using the `llms.sh --add` command.
+
+Preview available extensions:
+
+```bash
+./llms.sh --add
+```
+
+Add a specific extension:
+
+```bash
+./llms.sh --add xmas
+```
+
+## Built-in Extensions
+
+You can copy any of these built-in extensions from the source [llms/extensions](https://github.com/ServiceStack/llms/tree/main/llms/extensions) directory - see [Built-in Extensions](/docs/extensions/built-in) for the list of built-in extensions you might want to include in your custom build.

content/docs/deployment/meta.json

@@ -4,6 +4,7 @@
   "pages": [
     "index",
     "docker",
+    "custom-build",
     "github-oauth"
   ]
 }

content/docs/v3.mdx

@@ -143,6 +143,8 @@ UI components are now registered and referenced as Global Vue components, which
 
 This approach allows [main.py](https://github.com/ServiceStack/llms/blob/main/llms/main.py) to retain a **lean functional core in a single file** whilst still being fully extensible and lays the foundation for **rapid development of new features** - both from the core team and external 3rd party extensions - enabling the community to extend llms.py in new unanticipated ways.
 
+For deployments requiring minimal footprint, the [Custom Build](/docs/deployment/custom-build) docs shows how to create a tailored distribution with only the specific extensions you need - perfect for CLI-only or lightweight API server deployments.
+
 ## Extensions System
 
 To keep the core lightweight while enabling limitless enhancements, we've implemented a flexible **Extensions system** inspired by ComfyUI Custom Nodes. This allows adding new features, pages and toolbar icons, register new provider implementations, extend, replace, and customize the UI with your own custom features, just by adding new extension folders.