Move javadoc documentation to a service (or gh-pages branch)

I was thinking about using http://javadoc.io/ instead of hosting here, because it allows to have a quick view to all the versions already added there. Nevertheless, it requires to have Maven artifact published (#504). If we don't go down that road, we can also prepare a gh-branch with the javadoc and named folders containing them.

This does not mean that the javadoc task shouldn't be tested.