[DOCS] Fix ReadTheDocs coverage: TOC gaps, Python autodoc, stale references

[yunlishao-vibe]

Task Summary

Wire up Python autodoc, fix broken/orphaned TOC entries, add missing BUILD.md to navigation, update stale file references in DEVELOPER_GUIDE, and stub out thin placeholder pages with real content.

Branch and PR Target

• All PRs for this issue should target the dev branch

Scope of Work

1. Fix index.rst TOC

• Add BUILD to the Development section (currently not linked despite being the most useful developer reference)
• Remove or add Carla_0915_Windows_Fixes — currently orphaned (not in TOC, unreachable on RTD)
• Verify all files listed in index.rst actually exist

2. Fix stale references in DEVELOPER_GUIDE.md

• References RS_CM_BuildConfig_r2021b.py, RS_CM_BuildConfig_r2019b.py, RealSimCarMakerSetup.py — these filenames no longer match the repo
• Update to reflect current CarMaker build scripts (RS_CM11_1_2_BuildConfig_2024a.py, RS_CM13_1_3_BuildConfig_2024a.py, patch_user_c.ps1)
• Update CommonLib section to reflect removal of MathLibrary.h (unused), RealSimCmHelper, ryml_all.hpp

3. Wire Python autodoc

Now that CommonLib/__init__.py exists (PR #133), autodoc can generate real API pages:

• Add docstrings to CommonLib/ConfigHelper.py, MsgHelper.py, SocketHelper.py, VehDataMsgDefs.py
• Replace doc/api/index.md stub with proper automodule directives
• Verify sphinx.ext.autodoc and sphinx.ext.napoleon produce clean output on RTD build

4. Fill thin placeholder pages

These are in the TOC but nearly empty:

• changelog.md (11 lines) — add real version history from git tags / merged PRs
• about.md (2 lines) — expand with project mission, funding, affiliations
• usage.md (32 lines) — expand with real launch examples
• tutorials/quickstart.md, tutorials/examples.md — add minimal working examples using tests/Python/SimpleEchoClient

5. Defer: C++ Doxygen API

C++ headers (ConfigHelper.h, SocketHelper.h, MsgHelper.h, TrafficHelper.h) have zero Doxygen comments. Adding breathe+exhale now would generate empty pages. This should be a follow-on issue once doc comments are added to the headers.

Files Involved

• doc/index.rst
• doc/DEVELOPER_GUIDE.md
• doc/api/index.md → replace with autodoc .rst files
• doc/changelog.md, doc/about.md, doc/usage.md
• doc/tutorials/quickstart.md, doc/tutorials/examples.md
• CommonLib/*.py (docstrings)
• doc/conf.py (if autodoc config needs updating)
• doc/requirements_doc.txt (no new deps needed for autodoc)

Acceptance Criteria

• RTD build passes with no warnings on missing files
• BUILD.md appears in RTD navigation
• Python API reference renders real content (not "forthcoming" stub)
• DEVELOPER_GUIDE references match actual filenames in repo
• No orphaned .md files outside the TOC