Configuration property references are rendered incorrectly in the reference documentation

[wilkinsona]

We're missing a dependency on the Asciidoctor extension so source like configprop:spring.jpa.hibernate.ddl-auto[] property is left as-is in the resulting documentation.

[wilkinsona]

Fixing this is a little harder to fix than I would like. The Asciidoctor Gradle Plugin handles all log records as warnings, irrespective of their level. This means that our convention of treating all warnings as fatal causes the Asciidoctor tasks to fail due to the extension logging debug messages when a configuration property is successfully validated. I've opened asciidoctor/asciidoctor-gradle-plugin#529. In the meantime, we'll have to use a custom regular expression to ignore the success messages.