Add links config for maven javadoc plugin

Our published Javadoc at https://www.onehippo.org/library/about/javadoc.html will only include proper cross-linking between projects if the maven javadoc plugin has extra config to define the URLs for each related project. For some reason, we have never added this to the projects themselves, and we should. The Javadoc is published in a well-known location for each release branch, so this should be easy enough.