docs: add bootstrap extension

docs: add bootstrap extension

To get Sphinx and Bootstrap to work well together, we need to massage
the output from Sphinx a bit. This adds an extension to do such changes,
based on work from here:

https://github.com/pydata/pydata-sphinx-theme

...However, because we don't ship as an external theme, we can't just do
things as a part of __init__.py, so instead we register an extension
that does the heavy lifting for us.

Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8399>