Add Controller Kotlin sample for form login

Issue gh-8172

Author: eleftherias

docs/manual/src/docs/asciidoc/_includes/servlet/authentication/unpwd/form.adoc

@@ -197,8 +197,8 @@ A minimal sample `LoginController` can be see below:
 
 .LoginController
 ====
-.src/main/java/example/LoginController.java
-[source,java]
+.Java
+[source,java,role="primary"]
 ----
 @Controller
 class LoginController {
@@ -208,4 +208,16 @@ class LoginController {
 	}
 }
 ----
+
+.Kotlin
+[source,kotlin,role="secondary"]
+----
+@Controller
+class LoginController {
+    @GetMapping("/login")
+    fun login(): String {
+        return "login"
+    }
+}
+----
 ====