Enable/disable ToC at will

To disable ToC add in the yaml frontmatter of docs:

toc: false

By default set to true.

Do not merge unless the following get merged first:

• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2602
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13310
• omnibus-gitlab!1825 (merged)
• https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/644