Move parent-child pipelines to downstream page

doc/api/job_artifacts.md

doc/api/jobs.md

doc/api/pipelines.md

doc/ci/jobs/job_control.md

doc/ci/pipelines/downstream_pipelines.md

doc/ci/pipelines/index.md

doc/ci/pipelines/job_artifacts.md

doc/ci/pipelines/parent_child_pipelines.md

doc/ci/pipelines/pipeline_architectures.md

doc/ci/pipelines/pipeline_efficiency.md

doc/ci/resource_groups/index.md

doc/ci/troubleshooting.md

doc/ci/yaml/index.md

doc/user/application_security/coverage_fuzzing/index.md

.../application_security/policies/scan-execution-policies.md

doc/user/project/settings/index.md