docs: link to upstream etnaviv

docs: link to upstream etnaviv

The Wiki in this repo has been removed, and the about section mentions
that this is laanwj's personal fork, and not the upstream. So let's
update the link to the upstream repo instead.

There's no wiki there either, but the main repo itself contains a readme
file with some docs. Most of this is pretty stale, but it seems better
than nothing.

In the longer run, we should probably create a page about etnaviv in the
drivers directory of these docs, and try to keep that up to date.

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