This brings us closer to having a test of mdlintall that's happy -- but
not 100%: there's still errors about checkboxes that look like links and
confuses mdl

The support page is the go-to page for the "how to get help" question,
which previously was hidden inside the TPA-RFC-2 standard.

This basically copies that part of the TPA-RFC-2 policy, but could
eventually diverge without needing to update the policy, for informal
stuff.

I started working on this page while trying to address
team#40382 ("document the triage process"), but I'm not
actually sure triage documentation should go there, as this page feels
more targeted at users, and only users.

howto/incident-response.md would seem like a better space for this.

Also, this is a toplevel page because it's more important than
everything else, so it warrants its own special space.

The instructions were saying to sync them, but they were actually out
of sync already.

This was already in the home page, but not prominent. Makes sense to
have it in the main sidebar, and since those are in sync...