TEMP: add rst-conversion scripts

TEMP: add rst-conversion scripts

This is just a temporary commit, adding the scripts that performs the
automated conversion of the docs. The next commit contains the results
of the conversion, and the commit following that removes these scripts
again.

To redo the conversion in the next commit, rebase interactively to edit
this commit and delete the next one, and run './update-docs.sh' from the
root directory. Then continue the rebasing, and resolve any conflicts
that might have occurred in the manual fixes on top. Finally, build the
documentation to ensure no further fixups are needed.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4630>