Docs: remove duplicated info on CI/CD examples

The CI/CD index lists some https://docs.gitlab.com/ee/ci/README.html#examples that should be in https://docs.gitlab.com/ee/ci/examples/README.html

• Remove the duplicated info in https://docs.gitlab.com/ee/ci/README.html#examples
• Reorganize the examples in https://docs.gitlab.com/ee/ci/examples/README.html (fewer headers)
• Move all articles in https://docs.gitlab.com/ee/articles/index.html#build-test-and-deploy-with-gitlab-ci-cd to https://docs.gitlab.com/ee/ci/examples/README.html