This page is better suited for people landing immediately from
CONTRIBUTING.md, or who haven't heard of the guide before. "About This
Guide" is still useful, but doesn't need to be the very first page.

- Update outdated chat platforms
- Link to the community page
- Suggest `git shortlog` for finding experts

This is the first question I get from nearly all contributors. So far
I've been giving links to individual issues, but they quickly go out of
date or get fixed, and then I have more work to do to help people find
an issue. Add some suggestions for work people can find themselves,
without having to first consult an expert.

This also moves the "Cloning and Building" stub to the bottom of the
page.

Thank you JohnTitor for finding these!

Co-authored-by: Yuki Okushi <jtitor@2k36.org>

It has links to the specific channel/stream for each team.