Document GitLab Geo with Object Storage

What does this MR do?

Add some brief documentation on how to configure Object Storage (e.g. S3) with GitLab Geo.

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
• 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#3944

added 1 commit

• 72e8b393 - Document GitLab Geo with Object Storage

Compare with previous version

added 1 commit

• b1ef9b9c - Document GitLab Geo with Object Storage

Compare with previous version

resolved all discussions

added 68 commits

• b1ef9b9c...409d5851 - 67 commits from branch master
• 21994ddd - Document GitLab Geo with Object Storage

Compare with previous version

@nick.thomas Is this somewhat what you had in mind?

assigned to @nick.thomas

assigned to @to1ne

changed the description

added 1465 commits

• 21994ddd...752657df - 1463 commits from branch master
• a8803a7d - Document GitLab Geo with Object Storage
• c3dc4c76 - Add some instructions on how to set up replication between buckets

Compare with previous version

resolved all discussions

marked the checklist item Changelog entry added, if necessary as completed

changed the description

@nick.thomas Please have another look.

assigned to @nick.thomas