Add admin instance management and system tools

- Implemented sync_dev_hosts.sh for managing local hosts entries for development.
- Created admin_instances.html for managing school instances, including creation and listing.
- Developed admin_system.html for core services management, including restart and backup operations.
- Added my_instance.html for users to view and manage their assigned instances.
- Introduced provision_instance.sh for provisioning new instances with Docker, including Nginx configuration and SSL certificate handling.

Website/templates/admin_dashboard.html

@@ -274,6 +274,10 @@
         <p><strong>Team verwalten:</strong> Bilder, Rolle und Arbeit der zwei Team-Cards zentral pflegen.</p>
         <a href="{{ url_for('admin_team') }}">Team Cards öffnen</a>
     </div>
+    <div class="quick-admin-link">
+        <p><strong>Schul-Instanzen:</strong> Neue Subdomain-Instanzen für Schulen aus dem Inventarsystem-Repository starten.</p>
+        <a href="{{ url_for('admin_instances') }}">Instanzen öffnen</a>
+    </div>
     <h2>Buchungsanfragen verwalten</h2>
     <div class="block-days-section">
         <h3>Kalender-Tage sperren</h3>