Implement ParentEntityHandler plugin system for extensible entity support

Replace hardcoded entity type checks with a plugin-based architecture using
PHP 8 attributes. This allows adding new parent entity types without modifying
core module files.

Changes:
- Add ParentEntityHandler attribute, interface, base class, and manager
- Create built-in handlers for taxonomy_term, user, and node entities
- Move Group support to site_structure_group submodule (fixes class not found
  error when Group module is not installed)
- Refactor SiteStructureSettingsForm to use handler manager
- Refactor SiteStructureMenuBlock to use handler manager
- Refactor SectionBreadcrumbBuilder to use handler manager
- Update site_structure.module to use handler manager for clearsSiteSection
- Update documentation and translations

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

modules/site_structure_group/site_structure_group.info.yml

@@ -0,0 +1,8 @@
+name: 'Site Structure Group Integration'
+type: module
+description: 'Provides Group entity support as parent type for Site Structure module.'
+package: 'Site Structure'
+core_version_requirement: ^10.3 || ^11
+dependencies:
+  - site_structure:site_structure
+  - group:group

modules/site_structure_group/src/Plugin/ParentEntityHandler/GroupHandler.php

@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\site_structure_group\Plugin\ParentEntityHandler;
+
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\group\Entity\GroupInterface;
+use Drupal\site_structure\Attribute\ParentEntityHandler;
+use Drupal\site_structure\ParentEntityHandler\ParentEntityHandlerBase;
+
+/**
+ * Handler for group entities.
+ *
+ * This handler is provided by the site_structure_group submodule and is only
+ * available when the Group module is installed.
+ */
+#[ParentEntityHandler(
+  id: 'group',
+  label: new TranslatableMarkup('Groups (group)'),
+  entity_type_id: 'group',
+  provider_module: 'group',
+  clears_site_section: TRUE,
+  sort_field: 'label',
+  weight: 15,
+)]
+class GroupHandler extends ParentEntityHandlerBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getEntityFromRoute(RouteMatchInterface $route_match): ?EntityInterface {
+    $group = $route_match->getParameter('group');
+
+    if (!$group instanceof GroupInterface) {
+      return NULL;
+    }
+
+    return $group;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function handlesEntity(EntityInterface $entity): bool {
+    return $entity instanceof GroupInterface;
+  }
+
+}