[DISCUSSION] PR Review Process / States / "How to find the PRs in the list that need review"

This is related to the broader discussion about organizing DataFusion's growth:
- https://github.com/apache/datafusion/issues/21036

As the number of open PRs grows, it is getting harder to quickly see which PRs need attention and how best to help.

For me personally, I struggle wiht:
- If I have time to review PRs, it is not obvious which open PRs are waiting on review and which are waiting to incorporate feedback
- If I have time to merge PRs, it is not obvious which already approved PRs are [waiting for the 24 hour period ](https://datafusion.apache.org/contributor-guide/index.html#pull-request-overview).

For example, I personally use these two searches to find waiting PRs
- [DF Approved PRs](https://github.com/apache/datafusion/pulls?q=is%3Apr+is%3Aopen+review%3Aapproved+-is%3Adraft) - currently has 21 open
- [DF Review PRs](https://github.com/apache/datafusion/pulls?q=is%3Apr+is%3Aopen+-review%3Aapproved+draft%3Afalse+-author%3Aalamb+sort%3Aupdated-desc) - currently has 186 open

I think the core issue is that the current way to find PRs mixes together many different states, which makes it harder to triage efficiently

Also, there is currently no documented "review priority".

## Describe the solution you'd like

I would like a clearer and more explicit way to find PRs that need community attention.

I think at the core we probably need some sort of triage process and "workflow" for PR review / merge attention. I am acutely aware that we have very little extra capacity for such a process with the existing committers, but I do wonder if having it might attract others who can help the reviews along

## Describe alternatives you've considered

For example, a PR priority definition might look like (not saying this is the right one, it is just a strawman)
- Regression Bugs (what used to work doesn't anymore)
- Bugs
- Performance Improvements
- New Features

For example, a PR review flow might look like this
```mermaid
flowchart LR
A[Needs Initial Review] --> B[Feedback Received]
B --> C[Feedback Addressed]
C --> B
C --> D[Approved, waiting]
D --> E[Approved Ready to Merge]
```

## Additional context

Start with the contributor guide's Pull Request Overview and the linked GitHub PR searches, then review the related discussion in issue #21036. Done would require an agreed triage workflow, explicit review or merge states and priorities, and documentation that lets contributors find PRs needing attention.

Written by the indexing model from the issue text.