Docs streams flows basics ktoso

Starting on docs of "just flow sink and source", the "simple things".
Been going rather slowly though, still very very WIP, don't review in depth yet

refs #16549
From the API side of things you only see `Source[T]` then anyway, you don't know if it has more map stages before or not, and it still has exactly one output. What rewording do you have in mind here?