Update repository mirroring documentation to reflect current state.

What does this MR do?

Repository Mirroring documentation has fallen behind the current state of the code base. The screenshots and workflows are out of date.

Primarily this is a refactor of the existing content. Reviews might find ways to improve the content.

Related issues

This merge request builds on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20397.

Required porting: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7397. Port closed.

After migrating content from EE to CE, it still didn't merge cleanly so opened new MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7790.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/4302.

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards