Move mirroring page to new subdirectory, split

What does this MR do?

Does the first, and most painful, part of CTRT work for https://docs.gitlab.com/ee/user/project/repository/repository_mirroring.html:

• Move into a subdirectory and rename to user/project/repository/mirror/index.md
• Carve out three concepts into hew pages in that same new directory:
  • mirror/pull.md
  • mirror/push.md
  • mirror/bidirectional.md

Revisions to the text are as minimal as humanly possible:

• Fix inbound and outbound links.
• As all of pull mirroring is PREMIUM, keep the tier on the H1 and remove it from the subheadings.
• As all of bidirectional mirroring is PREMIUM, keep the tier on the H1 and remove it from the subheadings.
• Push mirroring is free, but I'm unsure if it's free or free-self, so I'll leave it for the follow-up. I don't want to introduce any NEW ambiguity here.

Prior to the split, this page had a reading score of grade 12.19. Unsurprisingly, the scores got (temporarily) higher when the pages got shorter:

• index.md: 12.52
• pull.md: 12.97
• push.md: 11.17
• bidirectional.md: 12.14

Related issues

Related to #340350 (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.