Merging multiple PRs into a single release note is unintuitive and clunky

[alice-i-cecile]

Many large features (or areas of related work) end up being done over multiple distinct PRs. When triaging, it's not uncommon for multiple of them to get the S-Needs-Release-Note label. However in the final document, we want to group them under a single heading.

If we simply delete one of the entries, rerunning the tool to catch new updates will add back the old merged PRs!

The current workflow is:

1. Delete the file and entry used for the PR(s) you're merging in.
2. Remove the entry from the _release-notes.toml file.
3. Edit the file for the PRs you're reusing (without changing the file name)
4. Remove the S-Needs-Release-Note label from the PR(s) that have been merged in so they don't get regenerated

[alice-i-cecile]

I think the best way to do this is to add a set of PR numbers to the metadata for each release note entry, explaining which PR they cover. I expect this work could be nicely tackled alongside #1350, as they both touch on the question of 'when should we regenerate notes".