Initial commit: Site Structure module for Drupal

Drupal module that provides hierarchical site structure management
with support for sections, categories, and content items. Includes
path aliases with tokens, breadcrumb integration, and admin interface.

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

config/schema/site_structure.schema.yml

@@ -0,0 +1,16 @@
+site_structure.settings:
+  type: config_object
+  label: 'Site Structure settings'
+  mapping:
+    allowed_parent_targets:
+      type: sequence
+      label: 'Allowed parent targets'
+      sequence:
+        type: mapping
+        mapping:
+          entity_type:
+            type: string
+            label: 'Entity type'
+          bundle:
+            type: string
+            label: 'Bundle'