Update reference docs to a more modern CSS style

This change upgrade the framework to consume the newly-published
docbook-reference-plugin version 0.2.0, which includes among other
improvements a revamped CSS approach designed to look as similar
as possible to the way markdown files are rendered by GitHub.

See associated commit [1] at SpringSource's gradle-plugins repo for
further details.

Issue: SPR-10036

[1]: b209c350ed

build.gradle

@@ -3,7 +3,7 @@ buildscript {
         maven { url 'http://repo.springsource.org/plugins-release' }
     }
     dependencies {
-        classpath 'org.springframework.build.gradle:docbook-reference-plugin:0.1.6'
+        classpath 'org.springframework.build.gradle:docbook-reference-plugin:0.2.0'
     }
 }