Documentation for "Add ability to duplicate the common metrics dashboard"

Problem to solve

We should document the results of changes at !21929 (merged) which correspond to issue #37238 (closed)

Docs at https://docs.gitlab.com/ee/user/project/integrations/prometheus.html#defining-custom-dashboards-per-project should be enhanced.

Further details

Proposal

Who can address the issue

Other links/references