doc: refresh review guide, note required reviews
Created by: bobheadxi
- Updates code review guidance to be inline with https://github.com/sourcegraph/sourcegraph/issues/29005 , turning our "doesn't require a review" stance into a "doesn't require an in-depth review" stance.
- Generalizes code reviews to "pull request reviews", since part of https://github.com/sourcegraph/sourcegraph/issues/29005 is that we enforce reviews on everything. I also believe a lot of this guidance applies whether you're making a doc change or a code change or anything else - I don't think we need to restrict this to code.
Closes https://github.com/sourcegraph/sourcegraph/issues/29005