Add CERN Infrastructure tools link in Confguration

Related to DRUPAL-266

cern_integration.links.menu.yml

+cern_integration.settings:
+  title: 'CERN Infrastructure'
+  parent: system.admin_config_system
+  description: 'Access CERN Drupal infrastructure tools (e.g., database administration, file access)'
+  route_name: cern_integration.settings

cern_integration.routing.yml

+cern_integration.settings:
+  path: '/_site'
+  defaults:
+    _title: 'CERN Infrastructure'
+  requirements:
+    _permission: 'administer site configuration'