Add design and UI changes checklist

What does this MR do and why?

When reviewing MRs, designers mostly focus on reviewing the visual design, content, and flow. There aren’t clear acceptance criteria.

This MR adds clear acceptance criteria to aid with designer’s reviews, mainly for MRs in the GitLab project, so that MRs are reviewed consistently.

See insight from the internal survey done for the FY22-Q3 KR: Improve the quality of design MR reviews.

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.

Follow-ups

• Remove duplicate content and link to new checklist: gitlab-com/www-gitlab-com!91000 (merged)