Asciidoc plumbing changes

Hi folks,

I bumped asciidoctorj to the latest version (3.0.1). The asciidocGroovyDSL
v2 module that we were using hasn't been ported to v3 yet. I have backed it
out and replaced its usage with an asciidoc-extensions module. This really
belongs in build-logic, but doesn't seem to package nicely in there, hence
the separate module. It isn't included in our standard install, mentioned
in poms like groovy-all, nor published to Maven, and if the Groovy DSL gets
ported, we can remove it. Just in case that change was confusing anyone.

Cheers, Paul.