Document sample use cases for each Flow state and other non-trivial use cases

Flow states like Wait, [Fail](https://github.com/Motiva-AI/stepwise/issues/16#issuecomment-1292696583), Choice, Parallel. And non-trivial use cases like [nested state machines](https://github.com/Motiva-AI/stepwise/issues/16#issuecomment-1292696583).