Unfortunately, it doesn't seem like there's a way to have sphinx copy
this without moving the files, becasue html_extra_path doesn't copy the
directory itself when given a directory, only files inside and
subdirectories.
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4630>
