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

[bast]

See the index file for this lesson: https://github.com/coderefinery/documentation/pull/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.