doc: update release checklist for new release notes workflow

[kmill]

This makes it reflect how we are writing release notes for 4.9.0, including how to handle the releases_drafts folder and how and when to update RELEASES.md.

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 5d2403535ad2213e3dfca8e1b1356df67f4e3ab5 --onto bedcbfcfeed429428c3e7f008b6984fc8c2b2b76. (2024-06-14 10:25:02)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase f531f4e5db8f402a015a69373af9a002fe754acd --onto 4ed79472af1f85b13c51e9ed77d15d3b4c40e3cc. (2024-07-09 21:46:17)