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 ("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.
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