Clean up 'reviews' docs, create new 'manage' page

What does this MR do?

@phikai and I have grumbled for quite some time that https://docs.gitlab.com/ee/user/project/merge_requests/reviews/ conflates two similar things:

• the things you can do when reviewing a merge request
• HOW to review a merge request

I've been asked to think bigger in my work on devopscreate docs, and as I was starting to check the subheadings on this page for https://gitlab.com/gitlab-org/technical-writing/-/issues/842, I realized … wait, I've been thinking about this split all wrong. It's not two pages in the user docs. It's a page in the user docs for the specific tasks, and a tutorial page for how to review a merge request.

I apologize in advance for how ugly this merge request will get. I'm doing a lot of splitting, rewriting, and re-homing of content.

Related issues

• Related to https://gitlab.com/gitlab-org/technical-writing/-/issues/842+
• Related to New tutorial: how to do a merge request review (!148910 - merged) where I'm moving the tutorial work to
• Related to Docs: split review page apart into user docs an... (#455147 - closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product tier badge under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.