Reinstate checkstyle for reference docs code snippets

This commit also ensures that checks are performed before the
application is rendered to get early feedback on errors.

src/checkstyle/checkstyle-suppressions.xml

@@ -12,6 +12,9 @@
 	<!-- JMH benchmarks -->
 	<suppress files="[\\/]src[\\/]jmh[\\/]java[\\/]org[\\/]springframework[\\/]" checks="JavadocVariable|JavadocStyle|InnerTypeLast"/>
 
+	<!-- Framework docs -->
+	<suppress files="org[\\/]springframework[\\/]docs[\\/].+" checks="IllegalImport" id="bannedJUnitJupiterImports" />
+
 	<!-- spring-aop -->
 	<suppress files="[\\/]src[\\/]main[\\/]java[\\/]org[\\/]aopalliance[\\/]" checks="IllegalImport" id="bannedImports" message="javax"/>