docs: update link to #zink

docs: update link to #zink

We have decided to follow what's happening with #dri-devel for #zink as
well, so let's update our link there as well.

While we're at it, let's switch the link to an anonymous link, because
we're not referring to it anywhere, so it doesn't need to be named.

Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10917>