Revert "Remove PDF backend"

This reverts commit 9f7d1ec.

Author: marcphilipp

documentation/documentation.gradle

@@ -1,3 +1,5 @@
+import org.apache.tools.ant.taskdefs.condition.Os
+
 buildscript {
 	dependencies {
 		// upgrade to latest jruby version due to a bugfix needed for Windows 10.
@@ -102,6 +104,12 @@ asciidoctor {
 
 	backends 'html5'
 
+	// The PDF backend currently does not work on Linux
+	// see https://github.com/junit-team/junit5/issues/1099
+	if (Os.isFamily(Os.FAMILY_WINDOWS) || Os.isFamily(Os.FAMILY_MAC)) {
+		backends 'pdf'
+	}
+
 	attributes	'jupiter-version': version,
 				'platform-version': platformVersion,
 				'vintage-version': vintageVersion,