Config Doc - Scan config groups that are outside of the module

For traditional config roots, we also need to scan the potential config
groups that are not in this module.

Fixes quarkusio#42815

(cherry picked from commit 5464841)

core/processor/src/main/java/io/quarkus/annotation/processor/documentation/config/scanner/ConfigAnnotationScanner.java

@@ -259,6 +259,17 @@ private void scanElement(List<ConfigAnnotationListener> listeners, DiscoveryRoot
 
                     if (resolvedType.isEnum()) {
                         handleEnum(listeners, resolvedType.unwrappedTypeElement());
+                    } else if (resolvedType.isClass()) {
+                        TypeElement unwrappedTypeElement = resolvedType.unwrappedTypeElement();
+                        if (utils.element().isAnnotationPresent(unwrappedTypeElement, Types.ANNOTATION_CONFIG_GROUP)
+                                && !isConfigGroupAlreadyHandled(unwrappedTypeElement)) {
+                            debug("Detected config group: " + resolvedType + " on field: "
+                                    + field, clazz);
+
+                            DiscoveryConfigGroup discoveryConfigGroup = applyRootListeners(
+                                    l -> l.onConfigGroup(unwrappedTypeElement));
+                            scanElement(listeners, discoveryConfigGroup, unwrappedTypeElement);
+                        }
                     }
 
                     debug("Detected enclosed field: " + field, clazz);