Include gen-class in Java interop page

[ericdallo]

Currently the java-interop page mention and have details about all java interop but gen-class, would be nice if it could have a section just mentioning gen-class with examples and possible behaviors.

There is already a compilation page mentioning some examples, we could improve it and link from the java-interop one maybe

Kind of related to https://github.com/clojure/clojure-site/issues/198?