create a new "support" page

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 (closed) ("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.