Merge request reviews page badly placed, unfindable

@phikai commented on a page I'd run into before, and had issues with: https://docs.gitlab.com/ee/user/discussions/#merge-request-reviews

• It's not in with the merge request docs
• It's buried as a subheading
• The title implies it's a generic discussions page, but most of it is related to merge requests.

Suggestion: split apart in some reasonable fashion, and move the appropriate parts in with the merge requests under a reasonable and SHORT file name.