Credit page to Code Review group

I think this page belongs to either Code Review or Source Code, but
it's definitely part of Create. Let's add metadata.

doc/development/diffs.md

 ---
-stage: none
-group: unassigned
+stage: Create
+group: Code Review
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
 ---