docs: remove webmaster article

This article is out-of-date, so let's instead forward to the "About
Mesa3D.org" article on the main website. This explains where to submit
merge requests for wanted changes.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6862>
3 files changed