DocC documentation for JavaKit and related modules

[DougGregor]

Our general user guide is somewhat of a "getting started" guide, but we should move toward a more comprehensive model for documentation, which involves integrating DocC into the build and migrating reference content over there. This issue covers adopting DocC and starting that migration, not necessarily writing new content (that can be covered by separate issues). We'd like this documentation to be available on the Swift Package Index as well.

[Taenerys]

hi @DougGregor ! may I take up this issue? I think it’d be a great way to get familiar with the codebase 👍

[ktoso]

Sure, please do. If you have any questions let us know here or on the forums.

A basic docc page to get us started would be good, and we'll likely want an article for javakit and for jextract, but we can fill in those later.

You could have a look at the https://github.com/swiftlang/swift-java/blob/main/USER_GUIDE.md to seed some contents into it.