Add some more detailed steps on reviewing

[jfrost-mo]

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