We need to regenerate the docs if any of the rst-files are changed. So
let's add a simple extension to generate a depfile, so meson will pick
up any changes needed.
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11494>
