Chore/cleanup

[markleader]

Summary

Documentation and repository housekeeping update that removes stale instructions, consolidates Windows oneAPI guidance into the main installation docs, and synchronizes version/changelog metadata for the 3.0.3 line. No solver algorithms or numerical routines were changed.

Changes

• Added Codex skill docs under .codex/skills/ and updated AGENTS.md guidance.
• Cleaned up docs build instructions and output paths:
  • Updated docs/README.md (make html, output in docs/_build/html).
  • Updated docs/source/developer_guide.rst, faq.rst, and quick_start.rst build commands (cmake --build build-* form).
• Removed docs/source/Win11-oneAPI_Walkthrough.rst and merged/expanded that content into docs/source/installation.rst under a dedicated Windows + oneAPI step-by-step section.
• Removed the deleted walkthrough from docs/source/index.rst toctree.
• Fixed stale sample filename in README.md (h2_o2.py).
• Updated release/version metadata to 3.0.3:
  • CMakeLists.txt
  • source/bind/python/cea/__init__.py
  • docs/source/conf.py
• Updated CHANGELOG.md entries (including PR references for recent items).
• Updated .gitignore to ignore source/bind/python/cea/samples/intro.py.

Testing

• Not run (changes are documentation/metadata/housekeeping only; no core numerical code changes).

Compatibility / Numerical behavior

• No expected changes to numerical results
• Expected changes (explain and provide validation)