Docs: Merge EE doc/ci to CE

doc/ci/ci_cd_for_external_repos/bitbucket_integration.md

doc/ci/ci_cd_for_external_repos/github_integration.md

...i/ci_cd_for_external_repos/img/bitbucket_app_password.png

doc/ci/ci_cd_for_external_repos/img/bitbucket_webhook.png

.../ci_cd_for_external_repos/img/ci_cd_for_external_repo.png

doc/ci/ci_cd_for_external_repos/img/external_repository.png

doc/ci/ci_cd_for_external_repos/img/github_omniauth.png

doc/ci/ci_cd_for_external_repos/img/github_omniauth_list.png

doc/ci/ci_cd_for_external_repos/img/github_push_webhook.png

doc/ci/ci_cd_for_external_repos/img/github_repo_list.png

doc/ci/ci_cd_for_external_repos/index.md

doc/ci/environments.md

doc/ci/examples/README.md

doc/ci/examples/browser_performance.md

doc/ci/examples/container_scanning.md

doc/ci/examples/dast.md

doc/ci/examples/dependency_scanning.md

doc/ci/examples/license_management.md

doc/ci/examples/sast.md

doc/ci/examples/sast_docker.md