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>

site_structure.module

@@ -51,9 +51,11 @@ function site_structure_entity_presave(EntityInterface $entity): void {
   }
 
   // Handle site section based on parent type.
-  // For user and group, we don't set field_site_section - the context is the entity itself.
-  if (in_array($parent_entity_type, ['user', 'group'])) {
-    // Clear field_site_section as the context is the user/group, not a site section.
+  // Some entity types (like user and group) act as context containers themselves.
+  /** @var \Drupal\site_structure\ParentEntityHandler\ParentEntityHandlerManagerInterface $handler_manager */
+  $handler_manager = \Drupal::service('plugin.manager.parent_entity_handler');
+  if ($handler_manager->clearsSiteSection($parent_entity_type)) {
+    // Clear field_site_section as the context is the parent entity, not a site section.
     $entity->set('field_site_section', NULL);
     return;
   }