Rename module site_structure → structural_pages and vocabulary site_section → site_sections

Renames the module from site_structure to structural_pages and pluralizes
the taxonomy vocabulary machine name from site_section to site_sections,
updating all config, PHP, translations, and documentation references
while preserving field_site_section, clears_site_section, and $site_section_id.

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

config/schema/structural_pages.schema.yml

@@ -0,0 +1,16 @@
+structural_pages.settings:
+  type: config_object
+  label: 'Structural Pages 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'