Document pull request triage

Looking at the triage documentation, I'm noticing that Pull request triage isn't documented yet.
I'm guessing that's partially because it's mostly done by maintainers, but it would be nice to have a place to put our guidelines.

Some ideas for what we could document:
- Using git blame to request author reviews
- When to request additional team reviews
- Specifically, at least @Mickeon has expressed interest in being involved in error message PRs. We could make it a policy to request review from `godotengine/documentation` when error messages and such are edited.
- But also when the code owners just don't perfectly line up
- When to remove review requests (e.g. when the author made a mistake on rebase)
- Which labels to use (although I suppose that mostly overlaps with the labels slapped on issues)
- Editing the PR description to link issues / proposals / connected PRs
- Requesting better PR descriptions, commit messages, or titles