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.

framework-docs/framework-docs.gradle

@@ -18,10 +18,6 @@ dependencies {
 	implementation("org.assertj:assertj-core")
 }
 
-checkstyle {
-	sourceSets = []
-}
-
 jar {
 	enabled = false
 }
@@ -128,6 +124,8 @@ asciidoctor {
 	}
 }
 
+asciidoctor.mustRunAfter "check"
+
 /**
  * Generate the Spring Framework Reference documentation from "src/docs/asciidoc"
  * in "build/docs/ref-docs/pdf".