Suggested improvements to documentation

I might try to create a PR but for now, just jotting down some ideas that might improve the documentation:
* https://wayang.apache.org/docs/api/javadocs seems to be in your main branch but not your deploy branch
* "Getting started" has a mixture of information, I'd suggest moving "Requirements" to "How to build Wayang" and I'd suggest moving "Configure Wayang" and "Cost Functions" to a new page perhaps entitled "Configuring Wayang" or "Further details".
* I'd add an "Abstraction" section (to match style in "Scalable Machine Learning") in "getting started" with the unary/binary/loop/source/sink descriptions and maybe a little bit on PlanBuilder.
* There probably needs to be a "Scalable Deep Learning" doc to cover DLModel/TensorFlow.