GitLab Pages documentation refactor
- Will have to check with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8463 for merge conflicts.
- Suport for Gitlab Pages serving HTTP and HTTPS on multiple IPs. https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/383
- Related to https://gitlab.com/gitlab-org/gitlab-ee/issues/461
- Mostly closes https://gitlab.com/gitlab-org/gitlab-ee/issues/465
- TODO in a separate MR: Refactor user docs.