Wiki: Add generate() examples to Creating-Observables.md

[akarnokd]

Wiki target:

https://github.com/ReactiveX/RxJava/blob/2.x/docs/Creating-Observables.md

See JavaDocs:

http://reactivex.io/RxJava/2.x/javadoc/io/reactivex/Flowable.html#generate-io.reactivex.functions.Consumer- +subsequent overloads

Related: #6132