Docs: remove duplicate CI examples

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

• For https://gitlab.com/gitlab-org/gitlab-ce/issues/41138
• Ports https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16372 to EE
• Copyedit EE-only part of https://docs.gitlab.com/ee/ci/examples/README.html