bpo-43795: PEP 652 user documentation (GH-25668) (GH-26034)

- Reformat the C API and ABI Versioning page (and extend/clarify a bit)
- Rewrite the stable ABI docs into a general text on C API Compatibility
- Add a list of Limited API contents, and notes for the individual items.
- Replace `Include/README.rst` with a link to a devguide page with the same info
(cherry picked from commit b05955d6f5f149523b5855a335444b7c6324bdb7)

Co-authored-by: Petr Viktorin <encukou@gmail.com>

Co-authored-by: Petr Viktorin <encukou@gmail.com>