Docs: remove duplicate CI examples

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

What does this MR do?

• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/41872
• For https://gitlab.com/gitlab-org/gitlab-ce/issues/41138
• Remove duplicate CI examples in https://docs.gitlab.com/ee/ci/README.html#examples (move them to https://docs.gitlab.com/ee/ci/examples/README.html)
• Moves all CI/CD articles in https://docs.gitlab.com/ee/articles/index.html#build-test-and-deploy-with-gitlab-ci-cd to the CI readme or to the CI examples, depending on the context

EE-version: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4008