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
We should talk about "bounded processing", i.e. bounded number of elements processed at every point in time. Explain that bounds are understood as bounds of element count, not element size (has consequences for streams of ByteString for example).