[docs] "build akka streamlets" section should start with a generator streamlet

[michaelpnash]

Easier to understand for the reader, to start from left to right, they can even deploy the generator by itself, since it has an outlet. In a hello world situation it is fine to have a ReportGenerator -> ReportPrinter blueprint.The example already talks about an ingress but never shows it, so it is hard to grok for newcomers.The suggestion is also not to add a ReportIngress with http, but just a generator, and explain that in a real app you would have some ingress. (this sidesteps all the complexity you have to explain about JSON, formats, Http, etc, while all you want to do is get through the mechanics of creating inlets, outlets, shape, and logic)