Adding UX as a requirement for "done" in merge requests

What does this MR do?

Add an additional criteria for "done" for contributions. Specifically, considering UX.

Does this MR meet the acceptance criteria?

Yes

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because Any docs-only changes should not have a changelog entry.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content