This commit ensures that only the HTML version of the reference
documentation is rendered for SNAPSHOT builds. This speeds up
significatly the build.
Issue: SPR-15885
This commit uses a tip provided by @mojavelinux to set the syntax
highlighter to Rouge for the PDF version of the User Guide.
Once Asciidoctor 1.5.7 has been released we will likely switch to Rouge
for the HTML version of the Reference Manual as well.
Issue: SPR-14997
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
