docs: Move the gallium driver documentation to the top level.

I actually had never found these, buried under Developer Topics -> Gallium
-> Drivers.  Given that driver documentation contains not just gallium
driver documentation but also end-user information, bring it to a much
more prominent location between User Topics and Developer Topics at the
top level.

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7174>
14 files changed