[PersistableState] Improve documentation for how to use persistable state services

[stacey-gammon]

This issue covers two things.

1. A new tutorial should be added to the developer guide to cover both how to persist persistable state in a saved object (very important to call inject/extract/migration functions), as well as how to implement persistable state (call out how important it is to write a migration function any time you change the shape of your data).
2. The in-code comments on the persistable state service could be improved or should direct the user to the tutorial for more information.

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)

[stacey-gammon]

Peter, can you add a menu navigation entry before closing this so it shows up in the dev guide?