Build a merge conflicts page from existing info

What does this MR do?

In prep for the work needed for #335341 (closed), these pages need to be blended together and given a new home:

• https://docs.gitlab.com/ee/topics/git/git_rebase.html#merge-conflicts
• https://docs.gitlab.com/ee/user/project/merge_requests/resolve_conflicts.html

Intended destination: doc/user/project/merge_requests/conflicts.md

Note: this MR won't be a complete CTRT rewrite. I'll get it as far as is feasible in this first pass, but I'm faaaaar past MVC at this point: https://about.gitlab.com/handbook/product/product-principles/#the-minimal-viable-change-mvc

After this work completes, I can then work with @pedroms etc to build in subheadings that talk about each of the merge conflict situations we now acknowledge in the UI. (Described in #335341 (closed).)

Related issues

Related to #335341 (closed)

• Related to #281171 (closed)

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are only adding documentation, do not add any of the following labels:

• ~"feature"
• ~"frontend"
• ~"backend"
• ~"bug"
• ~"database"

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

Review 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.
  • 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 above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.