DATAREST-364: Converted existing docbook reference material to asciidoct...

...or

Using https://github.com/gregturn/docbook2asciidoctor, converted all the source material to asciidoctor. Tested locally with CLI asciidoctor. Then tested using spring-data-build's `mvn -Pdistribute generate-resources`. Verified things look good.
This and the two following settings can go as they're defined by the build step externally.