Add user documentation for NAV dashboards

[lunkwill42]

Is your feature request related to a problem? Please describe.

There is no written documentation that explains to new users how dashboards work. We've made several changes to dashboard functionality recently, and we should probably write some documentation while we're at it.

Describe the solution you'd like

There is a user guide that starts with showing how to set up NAV, how to add stuff to NAV and how to organize the user's data. The guide stops after that. A next natural step might be a tutorial or guide to dashboards.

The whole dashboard concept should be explained:

• Each user can configure any number of personal dashboards (but there is always at least 1).
• Default dashboards
• Dashboard navigation (clicking on dashboard name, or arrow-key navigation)
• Placement and configuration of widgets.
• Full-screen and compact/non-compact toggles
• Dashboard definitions can be exported and imported
• Dashboards can be shared (with the caveat that when subscribed to someone else's dashboard, you are at their mercy when it comes to the content of that dashboard)