Add some more detailed steps on reviewing

### What problem does your feature request solve?

The reviewing documentation could be improved.

### Describe the solution you'd like

Something like https://code.metoffice.gov.uk/trac/rmed/wiki/dev/practices

Probably also want to link to the GitHub docs on making a review: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/reviewing-proposed-changes-in-a-pull-request

### Describe alternatives you've considered

Locate the existing reviewing documentation in CSET, then compare its coverage with the linked Met Office practices page and GitHub's reviewing-proposed-changes guide. Done means the documentation contains clear, detailed review steps and links to the relevant guidance.

Written by the indexing model from the issue text.