Add documentation to PipelineArtifacts

This commit adds a new page to the documentation explaning
what a pipeline artifact is, how is it used and where is it
persisted.

doc/ci/pipelines/pipeline_artifacts.md

+---
+stage: Verify
+group: Continuous Integration
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+type: reference, howto
+---
+
+# Pipeline artifacts
+
+Pipeline artifacts are files created by GitLab after a pipeline finishes. These are different than [job artifacts](job_artifacts.md) because they are not explicitly managed by the `.gitlab-ci.yml` definitions.
+
+Pipeline artifacts are used by the [test coverage visualization feature](../../user/project/merge_requests/test_coverage_visualization.md) to collect coverage information. It uses the [`artifacts: reports`](../yaml/README.md#artifactsreports) CI/CD keyword.
+
+## Storage
+
+Pipeline artifacts are saved to disk or object storage. They count towards a project's [storage usage quota](../../user/group/index.md#storage-usage-quota). The **Artifacts** on the usage quote page is the sum of all job artifacts and pipeline artifacts.
+
+Pipeline artifacts are erased after one week.