XD-167 - Generate reference doc from Wiki

Added several Gradle tasks:

* pullDocs (Pull wiki via Gradle Git)
  - The Git operation will be ignored if you work offline (`--offline`)
* asciidoctorDocbook (depends on pullDocs)
* asciidoctorHtml  (depends on pullDocs)
* reference (referencePdf [depends on asciidoctorDocbook], referenceHtmlMulti, referenceHtmlSingle