Do not render PDF reference doc for SNAPSHOT builds

This commit ensures that only the HTML version of the reference
documentation is rendered for SNAPSHOT builds. This speeds up
significatly the build.

Issue: SPR-15885

gradle/docs.gradle

@@ -54,7 +54,11 @@ asciidoctor {
 		include '*.adoc'
 	}
 	logDocuments = true
-	backends = ["html5", "pdf"]
+	backends = ["html5"]
+	// only ouput PDF documentation for non-SNAPSHOT builds
+	if(!project.getVersion().toString().contains("BUILD-SNAPSHOT")) {
+		backends += "pdf"
+	}
 	options doctype: 'book', eruby: 'erubis'
 	attributes  'icons': 'font',
 			'idprefix': '',