Switch from m2r to sphinx-mdinclude

[qmonnet]

m2r is unmaintained. It relies on mistune, but is not compatible with mistune versions >= 2.0. This makes it impossible to update mistune, even though there is a CVE in the old versions of the package.

Switch to sphinx-mdinclude instead.

Note: There is also m2r2, but it is not look like a suitable alternative because the project simply forces the use of an older mistune version.

I haven't managed to pass the full CI locally successfully, but the issues seemed to be related to my setup and not to the changes from this PR. If they did come from the PR, I would welcome some assistance to address them.

Fixes: #123 Possibly addresses #121 as well