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
I’m not sure what the pit is here: the type returned is not “keyed”, so it should be fairly obvious what is going on, or not? “Pitfall” implies surprising behavior, and I’m trying to understand where the surprise is.