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
Okay, so let’s make sure that that creation method (for the compound Sink in this case) is clearly documented to not return a KeyedSink—ScalaDoc tooltip now works even in Eclipse. Then we can also mention it here, but we should do our best to make it not surprising in the first place.