DATAREST-364: Converted existing docbook reference material to asciidoctor

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.

* Stripped out redundant line breaks
* Removed inline coding of titles
* Fine tuned level offsets