Document Geo HA

What does this MR do?

It documents how to set up Geo in a HA environment.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [-] API support added
• [-] Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • [-] Has been reviewed by Backend
  • [-] Has been reviewed by Database
  • Has been reviewed by Documentation
• 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 gitlab-org/gitlab-ee#3646 & gitlab-org/gitlab-ee#4061