[DOCS] Revisit all core FIXS codebase to enable readthedocs API support #37

Describe the issue
The core FIXS code (.py, .cpp, .m) has almost no doc comments, so Read the Docs can't generate an API reference. doc/api/index.md is still a "forthcoming" stub.

Revised 2026-09-23 after #137 / PR #395. The docs site now builds clean with warnings as errors (RTD fail_on_warning plus the docs.yml PR check), so this can land in small, verifiable pieces, Python first.

Suggested change

Phase 1: Python CommonLib (do first)

• Add Google-style docstrings (rendered by sphinx.ext.napoleon, already enabled) to CommonLib/ConfigHelper.py, MsgHelper.py, SocketHelper.py and VehDataMsgDefs.py. That's about 1,340 lines and 54 functions and classes, with almost no docstrings today:
  • a module docstring for each file
  • every public class and function
  • dataclass fields documented with units where the code or the C++ header (CommonLib/VehDataMsgDefs.h) establishes them
• Docstring-only change: prove no code moved by comparing each file's AST with docstrings stripped, before and after.
• Replace the doc/api/index.md stub with automodule pages for the four modules. Set autodoc_mock_imports = ["yaml", "numpy"] in doc/conf.py, because RTD installs only doc/requirements_doc.txt.
• Build stays warning-free (python -m sphinx -W --keep-going -b html doc _build/html; the PR check enforces it).

Phase 2: C++ headers (later)

• Doxygen comments on the public headers (ConfigHelper.h, SocketHelper.h, MsgHelper.h, TrafficHelper.h), then breathe/exhale. Adding the tooling first would only produce empty pages.

Phase 3: MATLAB (later)

• Decide on a format for the .m helpers (RealSimPack.m, RealSimDepack.m, ...) if API docs are wanted there.

Additional context

• How docs are built and published: doc/DEVELOPER_GUIDE.md → Documentation (Read the Docs).
• Related: [DOCS] ReadTheDocs: fix TOC/autodoc/stale refs + auto-build on main #137 (RTD cleanup), [DOCS] Python test documentation for ReadTheDocs and co-simulation message flow diagrams #122 (Python test tutorials).