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 will need proper definitions for "atomic" (indivisible) stream processing element. This is particularly important since there is a simple pitfall I discovered while writing the recipes: If you build up a Sink from an existing atomic Sink by prepending a flow, it still looks like a Sink but you cannot use it anymore as a key to the MaterializedMap. This must be explained somehow.