Review CI documentation

Added ~15078 label

@ayufan @axil we have separate CI section at http://doc.gitlab.com/. It is confusing because it uses deprecated repository. All new CI documentation should be located under http://doc.gitlab.com/ce/

https://gitlab.com/gitlab-com/doc-gitlab-com/issues/39 - related

@ayufan We need to remove it sooner that later to prevent people using outdated documentation.

Can you create a merge request to remove it from doc.gitlab.com?

mentioned in issue gitlab-com/doc-gitlab-com#39 (closed)

@dzaporozhets gitlab-com/doc-gitlab-com!18 (merged) should be good to merge.

Closing this in favor of #3260 (closed)

Status changed to closed

@axil thank you

mentioned in merge request !1755 (merged)