Add more documentation

[Ladicek]

The existing Jandex documentation consists of a README, which is even somewhat outdated, and a Javadoc. This needs to be improved quite a bit. I'd personally downplay the published Javadoc (it's enough you have access to it in an IDE) and instead focus on a user guide kind of thing. We could use the SmallRye documentation site (https://smallrye.io/docs/), as we plan to move to SmallRye anyway (#124).

[Ladicek]

The Jandex documentation should also really include documentation of the Maven plugin. All the more important that we merge the Jandex Maven plugin into the main project.

[n1hility]

FYI the javadoc is published to github pages atm: http://docs.wildfly.org/jandex/

[ljnelson]

(Do note the published javadoc is not for the currently released version (2.4.0.Final), however. As an arbitrary example, note the lack of ModuleInfo in the javadoc.)

[n1hility]

@ljnelson good point. I just pushed an update.

[Ladicek]

Thanks @n1hility! I knew it's published on GitHub Pages, but I don't really know how that's done, so I skipped it during the release :-)

[Ladicek]

I created a simple AsciiDoc/Antora-based documentation site in #242. It's currently almost equivalent to what used to be in the README, but I closed this issue anyway, as documentation is now much easier to add (and general docs improvements are always a long-running kind of thing). We'll refer to javadoc.io for javadocs, as we publish -javadoc.jar starting with Jandex 3.0 (actually 3.0.0-RC2 is already in Central and https://javadoc.io/doc/io.smallrye/jandex/latest/index.html works :-) ).