Documentation update: setting up backend database for use with dashboard

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [x] feature request

### Issue description

Would be brilliant to have a walkthrough on how to take the starter kit or the admin template, and set it up with a backend database / API layer for populating live data into the graphs and other elements. Appreciate there are many ways to set up databases to interoperate but a sample of live data flowing through and tweaking of the ngx-admin template to easily allows this modification in the data.service.ts class instead of having static data would be a really welcome addition for people looking to operationalise this