Add dependency injection doc

[snapsl]

Make riverpod onboarding easier.

A great feature of riverpod is that we can easily implement dependency injection. But this is not described in the documentation.

The documentation should contain:

How-To implement dependency injection using riverpod:

• provider (singleton)
• ProviderScope override
• family (I am not sure if that is desired)
• testing

If needed, I can contribute to the docs.