RADV is the last driver in Mesa that doesn't have a webpage somewhere
that we can link to from mesa3d.org. So let's make a minimal docs article
for it where we link to relevant information elsewhere.
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15494>
