Revisit documentation generation process with newer tools

While generating docs for the release, we leverage Gradle.
Newer version of Gradle apparently require JDK 9+ but if provided, then our ancient asciidoctor-maven-plugin fails. Upgrading this plugin in turn yields errors in doc formatting.

We should, in general, investigate generating docs on newer Javas and upgrade plugin and docs themselves accordingly. Note that you can generate docs regardless of Gradle of course and so it should be reproducible just by using JDK 9+.