docs: use html_static_path for static files

docs: use html_static_path for static files

I'm not sure why I picked html_extra_path instead, as it's meant for
slightly less directly related files than what html_static_path is for.

So let's switch. It shouldn't make much of a real-world difference, but
should make it a bit easier to understand what this is about.

Reviewed-by: Corentin Noël <corentin.noel@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25585>