akka streams docs: graphs

WIP on graphs parts of docs.

refs https://github.com/akka/akka/issues/16549
I could do the entire "max of 3" as one ZipWith, and expose 3 inputs - would be the nicest I think.
But I first need to finish and merge https://github.com/akka/akka/issues/16464 then.

Or I can do the same pipeline as already is but using ZipWith (lesser gain). Maybe let's delay updating this one until "generalized ZipWith" is merged and in that PR I'll also update this doc?