Docs: Enhance app plugin documentation, esp. storing configurations

[josmperez]

Feedback from Erik:

"An article describing how to store configurations in an app plugin would be helpful."

[academo]

This is already partially addressed here https://grafana.com/developers/plugin-tools/how-to-guides/app-plugins/add-authentication-for-app-plugins#encrypt-secrets-configuration

I am concerned that if we try to split the documentation on very dedicated topics we will end up with either duplicated documentation that is hard to maintain or too scattered for people to follow through

[josmperez]

@sunker do you think that the discussion of secrets configuration linked above in Esteban's comment adequately addresses your concern with existing app plugin docs?