docs: refresh user and API entry points

[MrFruitDude]

Summary

• refresh the top-level user and reference documentation landing pages
• add focused C++ and Python API overview pages and expand the documented API surface
• update the getting started, overview, release notes, and plugin appendix pages to remove stale wording

Validation

• git diff --check -- README.md docs/index.rst docs/reference/api/index.rst docs/reference/api/plugin.rst docs/reference/api/overview.rst docs/reference/build_guides/index.rst docs/reference/index.rst docs/reference/python_api/api.rst docs/reference/python_api/index.rst docs/reference/python_api/overview.rst docs/user_docs/appendix/python_plugins.rst docs/user_docs/getting_started/getting_started.rst docs/user_docs/index.rst docs/user_docs/overview.rst docs/user_docs/release_notes/index.rst
• custom python validation for duplicate labels, toctree targets, and stale entry-page wording

Notes

• full Sphinx/Doxygen generation was not run because sphinx-build and doxygen are not installed in this environment