Add documentation and examples for configuring cloud storage for registry images.

What does this MR do?

Adds documentation and examples for configuring cloud storage for registry images.

Waits for backend config to get merged in: https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/816

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1320

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

cc\ @marin