gallium/docs: prefix exts dir with underscore

It's generally considered good practice to use underscore-prefixes for
directories that contains non-doumentation files, so let's do this for
our custom extensions as well.

Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Acked-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5691>
2 files changed