Split Backporting into "policy" and "guide" #1831

As a follow-up to #1819:

It would be great to have two sections for Backporting: a policy document to explain what's required (main first, backport with the [3.x] prefix, what kinds of changes get backported where -- perhaps the Development Cycle page is a good place for this), and a guide for the practicalities & automation (tags, bots, cherry-picker, Git commands).