[new feature] TestGrid DashboardGroup "Overview"

[Ethan Blackwelder]

Hi TestGrid users,

We're pleased to announce a new DashboardGroup Overview tab that summarizes all the dashboards in a dashboard_group. For example: https://testgrid.k8s.io/sig-release

These Overview pages can be accessed by clicking on the dashboard_group box on the TestGrid home page OR by clicking the brand new top-level tab (named after the dashboard_group). Feel free to bookmark them.

All dashboards that are configured in dashboard_groups have automatically been opted in to the new feature; no additional configuration necessary. For dashboards that don't exist within a dashboard_group, there will be no visual or behavioral changes (since there isn't an additional top-level tab bar in those cases).

Questions or issues? Ask on #sig-testing Slack channel.

Thanks!