Merge request main page: blend subheadings together

What does this MR do?

In !161247 (merged), I'm trying to achieve good positioning for some new content that doesn't have a good home. https://docs.gitlab.com/ee/user/project/merge_requests/ lacks a subheading about merging a merge request, but before I stack on new content, I'd like to reclaim some vertical space by combining some existing subheadings that don't provide a lot of value.

This merge request makes two sets of changes to https://docs.gitlab.com/ee/user/project/merge_requests/:

• The subheading View merge requests had three tiny child subheadings. (Assigned to you, For a project, For all projects in a group). I've made those tabs.

• The subheading Assign a user to a merge request had a child subheading Assign multiple users that was virtually a duplicate. We can replace this entire subheading by editing one line:

  Search for the user you want to assign, and select the user. GitLab Free allows one
  assignee per merge request, but GitLab Premium and GitLab Ultimate allow multiple assignees:

Related issues

Related to Document user responsibility when merging MR (!161247 - merged) where I'm trying to achieve good positioning for some content that doesn't have a good home.

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 availability details 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.