Link Adventure JavaDocs

[Trigary]

Now that Adventure is a central part of the Paper API, I feel like adding a link to it to the Javadocs page would be in order. While the Adventure JavaDocs are linked from the Paper Javadocs, I feel like the ability to directly open the Adventure JavaDocs would be helpful, at least that's my personal opinion.

One could argue that then why don't we add a link to each dependency's JavaDocs to the webpage. Fair argument, my only response is that Adventure plays a central lore (it is required in most UI/UX scenarios), while other dependencies are more like utilities or their scope is very limited.

[aurorasmiles]

In general, as already mentioned in #73, I think Adventure should be linked in the https://github.com/PaperMC/PaperDocs
However linking the JavaDocs on that page would probably still be a good idea.

[Trigary]

There were only 3 open issues and I managed to create a duplicate one, wow. Apologies.