[hibernate-dev] Asciidoctor and Gradle

I just pushed the initial work getting Asciidoctor building through Gradle. I have done zero styling or layout etc. This is just the initial ability to generate Asciidoctor docs via Gradle. At the moment there is just one doc (the ServiceRegistry guide that is still WiP) that is rendered to just HTML. If you want to give it a try, cd into the documentation directory and execute `gradle generateRegistryGuideHtml`. The output for now goes into target/asciidoc/topical/html