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
Yeah, having a tough time to find a name for all our junction/source/etc. @rkuhn suggested at some point processing stage, but it's overlapping with our `Stage`, so may cause confusion. /me in dire need of better name here :)