Split Backporting into "policy" and "guide"

[encukou]

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).

[BHUVANSH855]

Hi! I'd be interested in working on this issue if it's still available.

My understanding is that the goal is to split the current backporting documentation into two parts:

• Backporting Policy – explaining what should be backported, when, and the overall workflow/policy.
• Backporting Guide – covering the practical steps such as labels, Miss Islington, cherry-picker, and the manual backport process.

If that aligns with the intended direction, I'd be happy to work on it.