Create introductory documentation and tutorial about Stage Group dashboards

In #664 (closed) we've added a dashboard per stage group.

We need to capture basic information about the dashboards, how they are built, and how they can be used.

We could add that documentation to doc/development in the main GitLab-rails repository. I think that's where people we're trying to reach will look for it first.

In a separate issue, we will cover documentation for more in-depth topics: #756 (closed)

To close this issue we need:

• documentation in doc/development: https://docs.gitlab.com/ee/development/stage_group_dashboards.html
• links in relevant places (monitoring handbook page and scalability readme)
• quick video tutorial: https://youtu.be/xB3gHlKCZpQ