docs: [[_TOC_]] (and more?) undocumented

Document the usage of [[_TOC_]] in https://docs.gitlab.com/ee/user/project/wiki/ and possibly in https://docs.gitlab.com/ee/user/markdown.html#wiki-specific-markdown.

There seems to be some info on stack overflow.

Introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2952 (GitLab 8.6!).

Old description

The [[_TOC_]] macro in GitLab wikis is not documented anywhere. I only found it on stack overflow by searching on the web. It seems to me it should be documented here.

I suspect many more secret extensions actually exist, is that possible?

• Documents Feature [[_TOC_]]
• Follow-up from: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2952

New doc or update?

• New documentation
• Update existing documentation