WIP: Add merge request review check-list

What does this MR do?

Add a first version of FE maintainers checklist to the new FE guide.

I think it would be a good idea to maintain such a check list, that then is copied by the maintainer when they start the review. What do you think?

Just before it comes up, I think adding it to the MR template would bloat the MR template. The only thing I would change in the default template.

  - [ ] Has been [reviewed by Frontend](https://docs.gitlab.com/ce/development/new_fe_guide/development/merge_request_review.html)

Why was this MR needed?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-com/marketing/general/issues/2153