Add AsyncValue documentation

[snapsl]

Make riverpod onboarding easier.

AsyncValue is a core feature of riverpod when we use AsyncNotifier / FutureProvider, but is not explained in the documentation. Currently new user have to look into the api reference or source code to get basic information.

The documentation should contain:

• what is AsyncValue
  • utiltiy class
  • AsyncData /AsyncLoading / AsyncError
• how to use AsyncValue
  • modifying an async value
  • .guard()
  • resolve async value (map / pattern matching / value / valueOrNull)

In case information is missing, feel free to add them.

If needed, I can contribute to the docs.