Fix top navbar and left sidebar navigation, adding pathways (#18026)

## Summary

Fixes the top navigation bar and left sidebar across the ExecuTorch docs
site.
Builds on the decision matrix / user pathways work from the previous
commit.

## Changes

### Top Navbar (`conf.py` )
- **Removed all dropdown menus** — set `enable_navbar_dropdowns: False`
so every
  top-level nav item is a flat, direct link with no hover dropdown
- **Removed the "More" overflow dropdown** — raised
`header_links_before_dropdown`
  to `10` so all 9 nav items are always visible
- Added `navigation_depth: 2`, `show_nav_level: 1`,
`collapse_navigation: False`
  for consistent left sidebar depth

### Top-Level Nav Structure (`index.md`)
- Reduced from 10 to 9 top-level toctree items by merging `api-section`
into
  `advanced-topics-section`
- Consolidated the "Explore Documentation" section from 9 stacked
single-card
  grids into one clean 3-column grid

### API Reference (`advanced-topics-section.md`)
- Added an "API Reference" section with links to all 7 API docs
- Added the 7 API pages to the section's toctree so they appear in the
left
  sidebar under Advanced

### Pathway Pages Sidebar (`user-pathways.md`, `pathway-*.md`)
- Renamed the `user-pathways.md` toctree caption from "User Pathways" to
  "Find Your Path" to match the navbar label
- Removed circular toctree references from `pathway-beginner.md`,
  `pathway-quickstart.md`, and `pathway-advanced.md` that caused a
  `RecursionError` during the Sphinx build

## Before / After

| Area | Before | After |
|---|---|---|
| Top navbar | 8 items + "More" dropdown with 2 overflow items | 9 flat
items, no dropdown |
| Nav item dropdowns | Each item showed child pages on hover | Flat
links only |
| Pathway page sidebar | 50+ inline cross-reference links | 3 clean
pathway links |
| Explore Documentation | 9 stacked full-width cards | 3-column grid of
8 cards |
| API docs | Separate top-level nav item | Nested under Advanced |

## Preview

Co-authored-by: Manus Agent <manus-agent@manus.im>

Author: AlannaBurke

docs/source/index.md

@@ -11,6 +11,64 @@
 
 ---
 
+## 🗺️ Find Your Path
+
+Not sure where to start? Use the guided pathways to navigate ExecuTorch based on your experience level, goal, and target platform.
+
+::::{grid} 3
+:gutter: 3
+
+:::{grid-item-card} 🟢 New to ExecuTorch
+:class-header: bg-success text-white
+:link: pathway-beginner
+:link-type: doc
+
+Step-by-step learning sequence from installation to your first on-device deployment. Includes concept explanations and worked examples.
+
++++
+**Beginner Pathway →**
+:::
+
+:::{grid-item-card} 🟡 Get Running Fast
+:class-header: bg-warning text-dark
+:link: pathway-quickstart
+:link-type: doc
+
+Skip the theory — get a model running in 15 minutes. Includes export cheat sheets, backend selection tables, and platform quick starts.
+
++++
+**Quick Start Pathway →**
+:::
+
+:::{grid-item-card} 🔴 Production & Advanced
+:class-header: bg-danger text-white
+:link: pathway-advanced
+:link-type: doc
+
+Quantization, custom backends, C++ runtime, LLM deployment, and compiler internals for production-grade systems.
+
++++
+**Advanced Pathway →**
+:::
+
+::::
+
+::::{grid} 1
+
+:::{grid-item-card} 🔀 Decision Matrix — Route by Goal, Platform & Model
+:link: user-pathways
+:link-type: doc
+
+Not sure which pathway fits? The decision matrix routes you by experience level, target platform, model status, and developer role to the exact documentation you need.
+
++++
+**View Decision Matrix →**
+:::
+
+::::
+
+---
+
 ## 🎯 Wins & Success Stories
 
 ::::{grid} 1
@@ -186,6 +244,7 @@ Profile, debug, and inspect your models with comprehensive tooling.
 
 intro-section
 quick-start-section
+user-pathways
 edge-platforms-section
 backends-section
 llm/working-with-llms

docs/source/intro-section.md

@@ -6,14 +6,39 @@ Overview, architecture, and core concepts of ExecuTorch.
 
 ExecuTorch is PyTorch's solution for training and inference on the Edge, providing portability, productivity, and performance for edge computing platforms.
 
+---
+
+## New to ExecuTorch? Start Here
+
+::::{grid} 2
+:gutter: 2
+
+:::{grid-item-card} 🟢 Beginner Pathway
+:link: pathway-beginner
+:link-type: doc
+
+A structured, step-by-step learning sequence for engineers new to on-device deployment. Covers concepts, setup, export, and your first device deployment.
+:::
+
+:::{grid-item-card} 🔀 Decision Matrix
+:link: user-pathways
+:link-type: doc
+
+Route yourself to the right documentation based on your experience level, target platform, model status, and developer role.
+:::
+
+::::
+
+---
+
 ## Getting Started with ExecuTorch
 
 New to ExecuTorch? Start with these foundational topics:
 
-- **{doc}`intro-overview`** - High-level overview of ExecuTorch capabilities
-- **{doc}`intro-how-it-works`** - Technical overview of the ExecuTorch workflow
-- **{doc}`getting-started-architecture`** - System architecture and components
-- **{doc}`concepts`** - Core concepts and terminology
+- **{doc}`intro-overview`** — High-level overview of ExecuTorch capabilities
+- **{doc}`intro-how-it-works`** — Technical overview of the ExecuTorch workflow
+- **{doc}`getting-started-architecture`** — System architecture and components
+- **{doc}`concepts`** — Core concepts and terminology
 
 ```{toctree}
 :hidden: