Move MR-specific subheadings and their images

What does this MR do?

https://docs.gitlab.com/ee/user/discussions/ has grown out of control over the years. The merge request comment experience is slowly diverging from comments in issues / epics / etc, and we have multiple discussion-related items that are unique to merge requests. These items should be moved to pages where they belong.

This merge request...

• Chops out subheadings from doc/user/discussions/index.md and moves them to:
  • doc/user/project/merge_requests/index.md
  • doc/user/project/merge_requests/changes.md
  • doc/user/project/merge_requests/commits.md
• It moves the images that belong to these subheadings
• It updates inbound links to these subheadings
• It updates outbound links from these subheadings

What this MR does NOT do

No cleanup has been done on these subheadings. This work is lift-and-shift only. After this work merges:

• the original Discussions page should get polished
• the content I'm moving here should be reviewed and polished. Separate, smaller MRs.

Related issues

• Related to Adds docs for commenting on files in a merge re... (!123011 - merged) which I merged for Phil this morning, which made me realize it's time to stop letting the Discussions page grow worse again
• That the page was a mess was taken for granted in Continue untangling Threads and Reviews pages (!60965 - merged) two years ago.
• Related to Merge request reviews page badly placed, unfind... (#327754 - closed) where similar work was handled some time ago
• Related to Start cleanup of "threads" page (!60870 - merged) where earlier rounds of this work was done two years ago.

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 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.