Begin revisions of merge requests API page

What does this MR do?

Begins the needed revisions to doc/api/merge_requests.md by creating a Response H3 subheading as a child of https://docs.gitlab.com/ee/api/merge_requests.html#get-single-mr.

This page needs FAR more work. In a subsequent MR, I will blend the contents of both of these subheadings into the appropriate part of this Response table:

• https://docs.gitlab.com/ee/api/merge_requests.html#single-merge-request-response-notes
• https://docs.gitlab.com/ee/api/merge_requests.html#merge-status

Related issues

• Related to Follow-up from "Introduce new detailed_merge_st... (#379607 - closed)
• Related to Introduce new detailed_merge_status to the merg... (!101724 - merged) where I made the decision to move this work to a follow-up
• Related to Introduce new detailed_merge_status to the API (#370856 - 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 or changing the main heading of the page (H1), ensure that the product tier badge is added.
• 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 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.