docs: drop news in favour of the introduction as index-page

docs: drop news in favour of the introduction as index-page

This kind of only makes sense once we have a separate home-page. But I
think this is a good way of showing why we should do this; Sphinx
doesn't support pagination, because it's not meant as a general-purpose
website framewrork. And for documentation, pagination is not really
something you need.

There's probably a lot more pages that should be moved into a separate
webpage, similar to this. In general, I think this should be done for
pages that don't relate to the source code too much, e.g isn't needed to
understand the code, or for instance explains how to get the source code.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4630>