DATAREDIS-336 - Move to asciidoc for reference documentation.

Add  asciidoctor plugin to gradle. Reformat and extract some content of files that could not be parsed. Added already converted files.
DATAREDIS-336: Fine tune build process and formatting

Remove old docbook content
DATAREDIS-336 - Copy styles to output dir.

Upgraded to Asciidoctor 1.5.0. We now cheat on gradle docbook-reference-plugin by directly copying files to reference work dir to bypass expanding files. Create an empty source folder so that the plugin does not fail.
DATAREDIS-336 - Improvements to reference documentation build.

Fixed Asciidoctor source files to avoid as many warnings as possible.

Switched to matching AsciidoctorJ version. Fixed doctype definition by making it an option and not an attribute. Switched from backend to backends (as recommended for Asciidoctor 1.5).
DATAREDIS-336 - Use Spring Data Build CSS and fix resources zip content.

We now depend on the Spring Data Build resources zip to refer to the centrally managed stylesheet. Fixed the Asciidoctor setup to copy the CSS from the shared resources into the target folder. Tweaked docsZip task to trigger docbook generation as well as PDF generation before zipping and corrected the includes directives to create the proper zip file structure.