Make site section vocabulary configurable and simplify settings form

Add a vocabulary selector to the settings form so the site section
vocabulary is no longer hardcoded. Remove node bundle checkboxes
(section_page/content_page are always included), keep user and group
as configurable parent targets. All hardcoded 'site_sections'
references in PHP replaced with dynamic config reads via helper.

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

config/schema/structural_pages.schema.yml

@@ -2,6 +2,9 @@ structural_pages.settings:
   type: config_object
   label: 'Structural Pages settings'
   mapping:
+    site_section_vocabulary:
+      type: string
+      label: 'Site section vocabulary'
     allowed_parent_targets:
       type: sequence
       label: 'Allowed parent targets'