Split Backporting into "policy" and "guide"

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).

审查当前的 Backporting 材料和 Development Cycle 页面，issue 将该页面确定为放置 policy 的可能位置。将必需的 workflow——先处理 main、使用 [3.x] 前缀，以及哪些更改会被 backport——与涵盖 tags、bots、cherry-picker 和 Git 命令的实践指南分开；当两个部分都清楚涵盖各自的主题时，即视为完成。