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.

[rrousselGit]

Can you be more specific about what you want here? Riverpod is not a DI package. And there are already docs explaining how to define a provider, override it, and test it.

[snapsl]

Of course, using riverpod for DI is just a part of it link.
And eventually we use the same concepts (provider, override, testing) that are already defined in the documentation.

I thought about creating an exemplary use case maybe a "case study" that shows how to implement it. Dependency injection is such a frequently applied pattern. Why not promote how easy it is to implement using riverpod?