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>

docs/DESIGN.md

@@ -276,6 +276,15 @@ site_structure/
 │   └── pt-br.po                         # Portuguese (Brazil) translation
 │
 ├── src/
+│   ├── Attribute/
+│   │   └── ParentEntityHandler.php      # Plugin attribute definition
+│   │
+│   ├── ParentEntityHandler/
+│   │   ├── ParentEntityHandlerInterface.php      # Handler interface
+│   │   ├── ParentEntityHandlerManagerInterface.php # Manager interface
+│   │   ├── ParentEntityHandlerBase.php           # Base implementation
+│   │   └── ParentEntityHandlerManager.php        # Plugin manager
+│   │
 │   ├── Breadcrumb/
 │   │   └── SectionBreadcrumbBuilder.php
 │   │
@@ -283,8 +292,19 @@ site_structure/
 │   │   └── SiteStructureSettingsForm.php
 │   │
 │   └── Plugin/
-│       └── Block/
-│           └── SiteStructureMenuBlock.php  # Dynamic menu block
+│       ├── Block/
+│       │   └── SiteStructureMenuBlock.php  # Dynamic menu block
+│       │
+│       └── ParentEntityHandler/            # Built-in handlers
+│           ├── TaxonomyTermHandler.php     # taxonomy_term handler
+│           ├── UserHandler.php             # user handler
+│           └── NodeHandler.php             # node handler
+│
+├── modules/
+│   └── site_structure_group/              # Group integration submodule
+│       ├── site_structure_group.info.yml
+│       └── src/Plugin/ParentEntityHandler/
+│           └── GroupHandler.php           # group handler
 │
 └── docs/
     └── DESIGN.md                        # This document
@@ -358,16 +378,81 @@ site_structure/
 
 ---
 
+## Parent Entity Handler Plugin System
+
+The module uses a plugin system to support different entity types as parents for content_page nodes. This allows for extensibility without modifying the core module.
+
+### Architecture
+
+The plugin system consists of:
+
+1. **Attribute**: `Drupal\site_structure\Attribute\ParentEntityHandler` - PHP 8 attribute for defining handlers
+2. **Interface**: `ParentEntityHandlerInterface` - Contract for all handlers
+3. **Base class**: `ParentEntityHandlerBase` - Common implementation
+4. **Manager**: `ParentEntityHandlerManager` - Plugin discovery and aggregation
+
+### Built-in Handlers
+
+| Handler | Entity Type | Clears Site Section | Sort Field | Bundle Restrictions |
+|---------|-------------|---------------------|------------|---------------------|
+| TaxonomyTermHandler | taxonomy_term | No | name | site_section |
+| UserHandler | user | Yes | name | - |
+| NodeHandler | node | No | title | content_page, section_page |
+
+### Creating Custom Handlers
+
+To add support for a new entity type, create a handler plugin:
+
+```php
+<?php
+
+namespace Drupal\my_module\Plugin\ParentEntityHandler;
+
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\site_structure\Attribute\ParentEntityHandler;
+use Drupal\site_structure\ParentEntityHandler\ParentEntityHandlerBase;
+
+#[ParentEntityHandler(
+  id: 'my_entity',
+  label: new TranslatableMarkup('My Entities'),
+  entity_type_id: 'my_entity',
+  clears_site_section: FALSE,
+  sort_field: 'title',
+)]
+class MyEntityHandler extends ParentEntityHandlerBase {
+  // Override methods as needed
+}
+```
+
+### Handler Attributes
+
+| Attribute | Type | Description |
+|-----------|------|-------------|
+| `id` | string | Unique plugin ID |
+| `label` | TranslatableMarkup | Human-readable label |
+| `entity_type_id` | string | Entity type this handler manages |
+| `provider_module` | string|null | Module required for availability |
+| `clears_site_section` | bool | Whether to clear field_site_section |
+| `sort_field` | string | Field for sorting entities |
+| `route_parameter` | string|null | Route parameter name (defaults to entity_type_id) |
+| `bundle_restrictions` | array | Specific bundles to handle |
+| `weight` | int | Handler priority (lower = first) |
+
+---
+
 ## Group Integration
 
-The module supports the Group module for content organization:
+Group support is provided via the `site_structure_group` submodule, which adds a handler for group entities. This submodule requires the Group module.
 
+**Features**:
 - Content pages can have groups as parent entities
 - When a content_page has a group as parent, the context is the group itself
 - Breadcrumbs show: Home > Group Name > Parent Nodes > Current Page
 - `field_site_section` is cleared for group-parented content (context is the group)
 - Per-section permissions can be implemented via Groups
 
+**Installation**: Enable the `site_structure_group` submodule after installing the Group module.
+
 **Configuration**: Enable group types in the settings form at `/admin/config/local-modules/site-structure`
 
 ---
@@ -394,3 +479,4 @@ The module supports the Group module for content organization:
 | 1.1.0 | - | Added dynamic_entity_reference support for multi-type parents |
 | 1.2.0 | - | Added user and group entity support as parent types |
 | 1.3.0 | - | Added Site Structure Menu block for dynamic hierarchical navigation |
+| 2.0.0 | - | Refactored to plugin system for parent entity handlers. Group support moved to submodule. |