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/install/structural_pages.settings.yml

@@ -1,10 +1,11 @@
+site_section_vocabulary: site_sections
 allowed_parent_targets:
-  - entity_type: node
-    bundle: content_page
-  - entity_type: node
-    bundle: section_page
   - entity_type: taxonomy_term
     bundle: site_sections
+  - entity_type: node
+    bundle: section_page
+  - entity_type: node
+    bundle: content_page
   - entity_type: user
     bundle: user
   - entity_type: group