Clarify documentation around when a specific kind of review is required

Nobody has claimed this yet.

### What problem does your feature request solve?

We should clarify the expectation for different levels of review, depending on the type of change. Currently this is lightly mentioned in the [git documentation](https://metoffice.github.io/CSET/contributing/git.html#review-steps) however it should be more thoroughly covered.

This will make it clearer what level of review people should ask for when making a PR, and better set expectations.

### Describe the solution you'd like

The obvious place for this is the [Code Review documentation](https://metoffice.github.io/CSET/contributing/code-review.html), which currently does not mention this at all.

The note in the [Pull Requests section](https://metoffice.github.io/CSET/contributing/git.html#pull-request) should also be removed now.

We might also want to consider additional tooling to help with requiring multiple approving reviews.

Start with the Code Review documentation at contributing/code-review.html and the review-steps and pull-request sections of contributing/git.html. Clarify which level of review is expected for each type of change, remove the duplicated note from the Pull Requests section, and leave any additional tooling as a separate consideration.

Written by the indexing model from the issue text.