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](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added~~
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] ~~API support added~~
- [ ] Tests
  - [ ] ~~Added for this feature/bug~~
  - [x] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

cc\ @marin

See merge request !4812
Signed-off-by: Rémy Coutable <remy@rymai.me>