[Telemetry] Add documentation about Application Usage

[afharo]

Some folks are asking to the telemetry team about how they can make use of or enable the Application Usage telemetry feature for their plugin.

Let's write some documentation to make it clear that it's automatic based on the KP routing and they don't need to set anything up on their end.

Additionally, let's use this issue to extend the docs of the plugin kibana_usage_collection. Maybe adding some links to other READMEs would be enough.

[elasticmachine]

Pinging @elastic/pulse (Team:Pulse)

[afharo]

Remember to add also documentation about the usage of the SavedObjects: #70475 (review)

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)