Move pipeline pages to ci/pipelines

doc/api/job_artifacts.md

doc/api/jobs.md

doc/api/pipelines.md

doc/ci/environments/deployment_safety.md

doc/ci/jobs/job_control.md

doc/ci/multi_project_pipelines.md

doc/ci/parent_child_pipelines.md

doc/ci/pipelines/img/multi_pipeline_mini_graph.gif

doc/ci/pipelines/img/multi_project_pipeline_graph.png

...ci/pipelines/img/parent_pipeline_graph_expanded_v12_6.png

doc/ci/pipelines/index.md

doc/ci/pipelines/job_artifacts.md

doc/ci/pipelines/multi_project_pipelines.md

doc/ci/pipelines/parent_child_pipelines.md

doc/ci/pipelines/pipeline_architectures.md

doc/ci/pipelines/pipeline_efficiency.md

doc/ci/triggers/index.md

doc/ci/troubleshooting.md

doc/ci/variables/index.md

doc/ci/yaml/index.md