Integrate with Javadoc

[zml2008]

We currently reference a bunch of Adventure classes, which have additional context in the JD. It would be helpful to link directly to those classes. There are a couple extensions for that:

• https://github.com/bluekeyes/sphinx-javalink
• https://github.com/12awsomeman34/sphinx-JDLinker

These both have some limitations that make them a bit awkward -- but we could tweak whichever one to produce something fully suited to our needs.