Fix top navbar and left sidebar navigation, adding pathways

[AlannaBurke]

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

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18026

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 3 New Failures

As of commit ca2ec4b with merge base 7824373 ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t bd2172d8793ade222ea21f46d110c8553f8c67e242ca2f221f5f92a814763b88 /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 6acbfc684827182deb57c206226ad72f8eac8afafead5bb0da6cffa163da1ead /exec failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_linear_model

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[psiddh]

Thanks for this change, I am testing the newly added the page : https://doc-previews.s3.us-east-1.amazonaws.com/pytorch/executorch/18026/user-pathways.html

Hope this will be useful to our user base , specillay the new beginners who look to onboard.
@nil-is-all Should we add these new pages explicitly to track the hits ? (on dashboard)

[nil-is-all]

Thanks for this change, I am testing the newly added the page : https://doc-previews.s3.us-east-1.amazonaws.com/pytorch/executorch/18026/user-pathways.html

Hope this will be useful to our user base , specillay the new beginners who look to onboard. @nil-is-all Should we add these new pages explicitly to track the hits ? (on dashboard)

@psiddh the new use-case specific pathway pages look great! Since the Beginner, Quick Start, and Advanced pathways are separate URLs, it will be tracked explicitly on the dashboard. Will add another widget to track page views and active users for Pathways