Add documentation and changelog

Add new page with full explanation
Add section and link on group page
Add changelog entry

doc/user/group/index.md

@@ -740,6 +740,10 @@ and issues) performed by your group members.
 
 With [GitLab Issue Analytics](issues_analytics/index.md), you can see a bar chart of the number of issues created each month in your groups.
 
+## Repositories analytics **(PREMIUM)**
+
+With [GitLab Repositories Analytics](repositories_analytics/index.md), you can download a CSV of the latest coverage data for all the projects in your group.
+
 ## Dependency Proxy **(PREMIUM)**
 
 Use GitLab as a [dependency proxy](../packages/dependency_proxy/index.md) for upstream Docker images.

doc/user/group/repositories_analytics/index.md

+---
+type: reference
+stage: Verify
+group: Analytics
+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
+---
+
+# Repositories Analytics **(PREMIUM)**
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/215104) in [GitLab Premium](https://about.gitlab.com/pricing/) 13.3.
+
+To get a CSV of the code coverage data for all of the projects in your group, go to your group's **Analytics > CI/CD** page
+
+Each row in the provided CSV includes:
+
+- The date when the coverage was calculated (at most one per day, from the latest run that day)
+- The name of the project
+- The name of the job that generated the coverage report
+- The coverage value
+
+<!-- ## Troubleshooting
+
+Include any troubleshooting steps that you can foresee. If you know beforehand what issues
+one might have when setting this up, or when something is changed, or on upgrading, it's
+important to describe those, too. Think of things that may go wrong and include them here.
+This is important to minimize requests for support, and to avoid doc comments with
+questions that you know someone might ask.
+
+Each scenario can be a third-level heading, e.g. `### Getting error message X`.
+If you have none to add when creating a doc, leave this section in place
+but commented out to help encourage others to add to it in the future. -->

ee/changelogs/unreleased/mf-group-coverage-data-download-frontend.yml

+---
+title: Add group code coverage download button
+merge_request: 37853
+author:
+type: added