Merge branch 'docs-ssot-ci-1' into 'master'

Docs: ssot ci/files

See merge request gitlab-org/gitlab-ce!28801

doc/ci/enable_or_disable_ci.md

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 # How to enable or disable GitLab CI/CD
 
 To effectively use GitLab CI/CD, you need a valid [`.gitlab-ci.yml`](yaml/README.md)

doc/ci/git_submodules.md

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 # Using Git submodules with GitLab CI
 
 > **Notes:**

doc/ci/junit_test_reports.md

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 # JUnit test reports
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/45318) in GitLab 11.2.

doc/ci/metrics_reports.md

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+type: reference
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 # Metrics Reports **[PREMIUM]**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/9788) in [GitLab Premium](https://about.gitlab.com/pricing) 11.10.

doc/ci/multi_project_pipelines.md

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+type: reference
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 # Multi-project pipelines **[PREMIUM]**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/2121) in

doc/ci/pipelines.md

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+type: reference
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 # Creating and using CI/CD pipelines
 
 > Introduced in GitLab 8.8.