docs: inline contents.rst into index.rst

docs: inline contents.rst into index.rst

The idea of having a single file containing the ToC is not really how
things are done in Sphinx, and kinda makes it harder to structure
documentation more naturally. This was just something I did to mirror
what we used to do for the old HTML-only version of the docs, to ease
the transition and to de-clutter index.rst.

Now that the transition is far behind us, and index.rst is much cleaner,
we can finally start inlining this.

In the long run, I expect most of these to be moved to separate "chapter
articles" that summarize what these topics are, and thus disappear from
here.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7709>