This commit removes docbook from the documentation toolchain and
instead makes use of asciidoctor to render the reference documentation
in HTML and PDF formats.
The main Gradle build has been refactored with the documentation tasks
and sniffer tasks extracted to their own gradle file in the "gradle"
folder.
A new asciidoctor Spring theme is also used to render the HTML5 backend.
Issue: SPR-14997
Rossen Stoyanchev
Brian Clozel