Revise the pull mirroring page for CTRT

What does this MR do?

This MR attempts a major revision of the pull mirroring page. (https://docs.gitlab.com/ee/user/project/repository/mirror/pull.html) Extra complexity: I will be out until October 7.

As part of #340350 (closed) (@rdickenson's report to @sean_carroll that the mirroring page was a mess) I've split the mirroring information into 4 pages:

• https://docs.gitlab.com/ee/user/project/repository/mirror/index.html
  • https://docs.gitlab.com/ee/user/project/repository/mirror/pull.html
  • https://docs.gitlab.com/ee/user/project/repository/mirror/push.html
  • https://docs.gitlab.com/ee/user/project/repository/mirror/bidirectional.html

Some notes for the review team:

• The diff will be ugly. Use a review app. You'll resent me less.
• It should go first to either @sean_carroll or @andr3 for a factual review. They'll probably assign it to someone on the team. That's fine. Please make sure I didn't bobble any facts.
• Sean / André, after your team has a chance to review it, please talk to @cnorris to determine the next step. I won't be here to know if the changes are substantial or small. If the changes are small, the revision can go straight to Suzanne Selhorn, because all CTRT OKR work needs to go through her. If they're big, Craig may want to route this MR to another TW first to resolve any technical issues before sending on to Suzanne for the tone and style edit.
• For Sean / André and any TW that touches this MR: you have my blessing, encouragement, and specific request to push commits to this MR. Whatever changes are needed, please just do them. I will have several large MRs like this one, and I would rather handle multiple followup requests rather than untangling suggestions while fixing conflicts.

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.