Delete ancient 'managing PRs' flowchart

- This diagram was really old and didn't reflect the current practices
  (a lot more things are automated now, hurray).
- Anything we replace it with will get outdated fast too.
- Also delete the diagramming docs since I don't believe we have made
  another diagram since this one which got out of date and forgotten
  about since it was a DrawIO SVG and not easily editable as code.
- If we want diagrams these days, we can use Mermaid which is
  diagrams-as-code.