GitLab

We're using gitlab, so "merge-request" is a more appropriate term.
Reviewed-by: Eric Engestrom <eric@engestrom.ch>

Brian Paul asked to no longer be listed as the Mesa3D.org webmaster, as
he no longer has the time to maintain it, and the maintainence has
effectively become a community effort anyway.

This page replaces the old webmaster.html page with some more up-to-date
information about the site and how to make changes to it.