Consider PR labels to make reviewing easier

### Is your feature request related to a problem or challenge?

Currently we're sitting around ~190 PRs open; would be nice if we can try make reviews easier to try reduce this amount.

### Describe the solution you'd like

One thing I can think of is introducing labels such as:

- `pending review` - Needs someone to take a look at this
- `pending author action` - Some required changes before we can progress
- Technically we could use `Changes requested` review for this but maybe this is easier to see? Or maybe its a softer version, such as asking clarification from author (not necessarily code changes)
- `needs discussion` - For those PRs which aren't clear cut and need discussion from both sides to progress

This might help reviewers identify which PRs need attention, especially the `needs discussion` ones as those PRs can last a while and get pushed out in favour of newer PRs (and can lead them to languish); and sometimes they might be marked as draft which can make them harder to see.

A downside is how we apply these labels; more manual action from reviewers probably doesn't help solve the issue at hand 😅

### Describe alternatives you've considered

Another thing to consider is making the GitHub action to close stale PRs more aggressive? Though I feel a good number of the open PRs are stale mainly because they haven't been reviewed so might feel unfair to close them aggressively 🤔

### Additional context

_No response_

Start by reviewing the current GitHub Action that closes stale pull requests and the repository's existing pull-request review workflow. Compare the proposed labels and application process, then define an agreed approach; done means the review workflow and label ownership are decided and documented.

Written by the indexing model from the issue text.