Document files/directories requiring backmerging to WP Core for major release

[getdave]

What?

Documents the files/paths that do/don't needing back merging to WP Core for a major release.

Why?

Currently release leads spend a lot of time answering "Should this be backported?" questions. By documenting the basics we relieve some burden on those individuals and make it easier for future release leads.

How?

• Document the high level scope for files that need back merging
• Document any exceptions

We should also bring bin/generate-php-sync-issue.mjs into line with whatever is documented here.

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

Flaky tests detected in e94967b.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/7609880717
📝 Reported issues:

• [Flaky Test] the event listener is removed when the element is removed #57983 in /test/e2e/specs/interactivity/directive-on-document.spec.ts

[Mamaduka]

Thank you, Dave! I think this is a good start.

[youknowriad]

nice work here :)

[getdave]

Merging and iterating. Any changes that come up in #57959 should be added to this list.