Official javadoc access for 1.x and 2.x

[akarnokd]

Currently, the link http://reactivex.io/RxJava/javadoc/ points to the 1.x documentation. Since the two library versions will live side-by-side for some years, we have to make sure their javadocs are accessible.

I suggest having links as

http://reactivex.io/RxJava/1.x/javadoc/ and
http://reactivex.io/RxJava/2.x/javadoc/

and if possible, have a redirect in http://reactivex.io/RxJava/javadoc/ for 1.x.

[stevegury]

👍

[ZacSweers]

👍

[akarnokd]

Closing via #4424.