Add hands-on tutorial for Kotlin Flow

[shipsywor]

trafficstars

StateFlow, SharedFlow, etc. A hands-on type tutorial on these Flow topic would be very interesting. The coroutines hands-on tutorial is great. A Flow one would be a great addition.

[ianbrandt]

See also https://github.com/Kotlin/kotlinx.coroutines/issues/2946 about adding SharedFlow and StateFlow to the Coroutines Guide as well.