[FEAT] Conformance Data: display a legend

[tbouffard]

Is your feature request related to a problem? Please describe.
The colors are these which are commonly used for "Conformance" analysis. But for someone which needs a reminder or knows nothing about this topic, it is hard to understand which data are presented in this demo.

Describe the solution you'd like
Display a legend to explain what the demo displays.
We already did this in the "Monitoring Demo" hosted in the examples repository: https://cdn.statically.io/gh/process-analytics/bpmn-visualization-examples/v0.27.1/demo/monitoring-all-process-instances/index.html