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 don't see it obvious. You need to store the original keyed sink in a val before you start attaching flows to it. I personally see it as something that can very easily surprise someone. Just because something throws a type error does not mean it is not a pitfall.