These aren't built, so they won't get copied out into the public folder
unless we put is inside the _extra folder.
This has been broken all since the conversion to Sphinx, whoops!
Reviewed-by: Eric Engestrom <eric@igalia.com>
Reviewed-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21956>
