Move javadoc to javadoc.io and rework the navigation menu

This is a follow-up of Githug PR #217 discussion.

garydgregory hinted the following:

Instead of keeping this list [site.xml javadoc reference], why not point to javadoc.io like we do here https://commons.apache.org/proper/commons-io/ ?

My preliminary take on this was:

according to I've seen in commons-io, it seems that the (commons-compress) download page would also need some work in order to list previous versions, together with the javadoc links.

So, while this seems a good idea (would reduce site maintenance, increase project visibility, etc.), it's not as straightforward as one would like it to be.