Update Sphinx to v6.x

[lmazuel]

Move Sphinx to v6.x (matches the doc website version).

Few notes:

• Supports only MD now as Readme input format (we don't have RST anymore anyway)
• Don't convert those MD to RST, let Sphinx deal with it himself
• Update Jinja in a few places to latest version
• Remove deprecated sphinx plugin for MD, and use current recommendation myst-parser

cc @scbedd

[scbedd]

#32066 This ended up being a messy upgrade to fix cmarkgfm. A ton more dependencies needed to be upgraded to get a clean run.

I can either merge that into this or merge this into that. Let me know! I have a mostly clean run aside from the sphinx updates that you've added here.

EDIT: the test timeout is due to this test for some reason on flask > 1 it just....hangs downloading.

[scbedd]

I'm going to do an adjusted cherry pick of this PR into mine. To get all the environments running properly I had to update a bunch of requirements here and there. Given that so many updated, I think it'll be easier to merge your sphinx conf changes into mine.

EDIT: cherry-picked commit