Add documentation on how to use and read the new dashboard

When the dashboards are finished. We should add new documentation on how to read it.

I think this should tie in to the documentation we currently have in: https://gitlab.com/gitlab-org/gitlab/blob/1ba3d52a6209712c0aee39f16e83e0fb8f9e2b55/doc/development/stage_group_dashboards.md#L1. It's probably worth splitting up that page.