Move MR pipeline docs to pipelines-related dir

app/assets/javascripts/commit/pipelines/pipelines_table.vue

app/serializers/merge_request_widget_entity.rb

...s/projects/_merge_request_merge_checks_settings.html.haml

doc/administration/reference_architectures/10k_users.md

doc/administration/reference_architectures/25k_users.md

doc/administration/reference_architectures/2k_users.md

doc/administration/reference_architectures/3k_users.md

doc/administration/reference_architectures/50k_users.md

doc/administration/reference_architectures/5k_users.md

doc/api/merge_requests.md

doc/api/merge_trains.md

doc/ci/environments/index.md

doc/ci/index.md

doc/ci/jobs/job_control.md

doc/ci/merge_request_pipelines/index.md

...e_request_pipelines/pipelines_for_merged_results/index.md

...elines/pipelines_for_merged_results/merge_trains/index.md

doc/ci/parent_child_pipelines.md

doc/ci/pipelines/img/merge_train_cancel_v12_0.png

doc/ci/pipelines/img/merge_train_failure.png