Add details on how to disable GitLab to the DR documentation

What does this MR do?

Documentation update for DR. When disabling the old primary, we should ensure (as much as we are able) that it will never come back.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Robust DR for %10.5. This is the best we can do while omnibus-gitlab#3058 is open, and we're not expecting that to be resolved in %10.5.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Has been reviewed by Geo
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #4417 (closed)