Add example to the Grafana plugin tutorial for creating a network map in Pixie

[nserrino]

Is your feature request related to a problem? Please describe.
Grafana users are interested in seeing a Service Graph example in our tutorial in addition to the Bar Chart and Timeseries example.

Describe the solution you'd like.
We should have a third example widget in our Grafana plugin tutorial for Node Graph, which is Grafana's version of a Graph in our system. https://grafana.com/docs/grafana/latest/visualizations/node-graph/