sphinx: use MyST for markdown rendering

This allows to have full support for all sphinx directives and link types.

Note: this diff also contains enabling the TODO extension, as it's a one liner, but the changes will be commited separately.

Test Plan

tox -e sphinx-dev

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Migrated from D4041 (view on Phabricator)

Why do we need markdown support in the docs?

! In !78 (closed), @vlorentz wrote: Why do we need markdown support in the docs?

To be able to use hackmd/codimd to collaborate on doc that can then be integrated into the real doc without conversion.

But aside from that, we //already// have markdown document in our docs, they are not being introduced by this change. What this change does is enable to use actual sphinx stuff from those docs rather than being limited to prose and external links.

Merge request was accepted

approved this merge request

Merge request was merged

closed

mentioned in issue swh/meta#2785 (closed)

mentioned in issue #4737