Issue #289: Document the most common use case

[iguitton]

• Documented 3 use cases (custom monitor, SNMP monitoring, and ping)
• Updated site.xml
• Moved the example available in configure-monitoring.md to grafana-service.md

[iguitton]

@NassimBtk, @bertysentry: please share your insights about these first 3 use cases.

[iguitton]

@bertysentry , @NassimBtk : I took your comments into account and added 2 new use cases.

[iguitton]

@bertysentry : a new version is ready for review.