Update code review approval guidelines

What does this MR do and why?

This updates the code review guidelines in our documentation to reflect updated change management segregation of duties requirements. Because we now require at least one approval from someone who has not contributed a commit to an MR, we are removing conflicting language encouraging maintainers to make small fixes before merging.

Describe in detail what your merge request does and why.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.