docs: Add a link to the linux kernel DRM docs under "Developer Topics"

It's a great source of documentation and replaces a lot of the stale
content on the DRI wiki.

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7258>
1 file changed