Resolve "Modify the Geo setup instructions to copy gitlab-secrets.json between primary and secondary"

What does this MR do?

Update documentation to take account of the need to replicate more secrets than just db_key_base for ~"Geo DR"

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

I'm not too sure about the mechanism - copy & paste between two SSH terminals. The HA documentation for the equivalent step is much shorter and leaves the mechanism up to the user: https://docs.gitlab.com/ee/administration/high_availability/gitlab.html#extra-configuration-for-additional-gitlab-application-servers

Why was this MR needed?

Required fix for ~"Geo DR"

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Review
  • Has been reviewed by Backend
• 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

What are the relevant issue numbers?

Closes #4253 (closed)