Updated code review guidelines for attention requests

What does this MR do and why?

This merge request updates the GitLab Code Review Guidelines to begin dogfooding the new attention requests feature.

Communication Plan

A detailed communication plan for communicating this change across the GitLab organization can be found in #356694 (closed). Highlights include:

• Announcements across various slack channels (#whats-happening-at-gitlab, #development, #eng-week-in-review)
• GitLab notifications (mentions to Engineering Managers and Maintainers)

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.