Add Kotlin support for "addGeneratedAnnotation" flag (#1115)

This adds logic so that the Kotlin code generator also supports the
"addGeneratedAnnotation" flag.

Also:
* Add Antora documentation
* Adjust names (generated-annotation -> add-generated-annotation)
* Adjust doc comments

docs/modules/pkl-gradle/partials/gradle-codegen-properties.adoc

@@ -44,6 +44,14 @@ Example: `implementSerializable = true` +
 Whether to generate classes that implement `java.io.Serializable`.
 ====
 
+.addGeneratedAnnotation: Property<Boolean>
+[%collapsible]
+====
+Default: `false` +
+Example: `addGeneratedAnnotation = true` +
+Whether to add the `org.pkl.config.java.Generated` annotation to generated types.
+====
+
 .renames: MapProperty<String, String>
 [%collapsible]
 ====