The index file in a Sphinx project should not be .rst

[bast]

See the index file for this lesson: #284

[bast]

It will be simpler to introduce Sphinx with "pure" md (as interpreted by MyST) instead of using md everywhere except for the index file.

[bast]

One complication is that sphinx-quickstart always generates an index.rst and we would have to change it to index.md afterwards.

[mmesiti]

This seems relevant: https://myst-parser.readthedocs.io/en/latest/syntax/organising_content.html, in particular:
https://myst-parser.readthedocs.io/en/latest/syntax/organising_content.html#using-toctree-to-include-other-documents-as-children

[mmesiti]

This should be addressed in #323. Close - with calm - if that's the case.