WIP: Restructure documentation

What does this MR do?

Breaks documentation links. Just a WIP to see how the structure will look like in the end.

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

Yes.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/3349

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

Milestone changed to %8.11

Milestone changed to %8.12

Status changed to closed

Split into small MRs tracked in #3349 (moved)