Add docs about loading heartbeat dashboard

[dedemorton]

@urso You might want to add a more interesting screen capture of Heartbeat data.

BTW, I use "dashboards" plural in case we add more later.

[dedemorton]

Adds dashboard content for item tracked in #3539 meta issue.

[tsg]

You can find a somehow more colorful dashboard screenshot here: #3531

Also, the new screenshot for kibana-created-indexes.png seems wrong to me. Perhaps you replaced by mistake?

[dedemorton]

@tsg I've replaced the Heartbeat dashboard. There were a few minor changes to the Kibana UI after you added the dashboard, but I think that having realistic looking data is more important than having the UI match up exactly (it's not too far off).

The other screen capture, kibana-created-indexes.png, probably looks wrong because I replaced an old screenshot (which showed how to set an index as the default) with a screenshot of the Discover page. The old screen shot was leftover from me copying files to use as a template. We don't tell users to set an index as the default anymore because that happens automatically. So the new screenshot is the right one...it just looks weird when you compare it to the old one.