Add versionadded block and update PHP configuration example with new ConfigBuilder

Author: ker0x

form/bootstrap5.rst

@@ -1,6 +1,10 @@
 Bootstrap 5 Form Theme
 ======================
 
+.. versionadded:: 5.3
+
+    The Bootstrap 5 Form Theme was introduce in Symfony 5.3.
+
 Symfony provides several ways of integrating Bootstrap into your application. The
 most straightforward way is to add the required ``<link>`` and ``<script>``
 elements in your templates (usually you only include them in the main layout
@@ -55,13 +59,13 @@ configuration:
     .. code-block:: php
 
         // config/packages/twig.php
-        $container->loadFromExtension('twig', [
-            'form_themes' => [
-                'bootstrap_5_layout.html.twig',
-            ],
+        use Symfony\Config\TwigConfig;
+
+        return static function(FrameworkConfig $twig) {
+            $twig->formThemes(['bootstrap_5_layout.html.twig']);
 
             // ...
-        ]);
+        };
 
 If you prefer to apply the Bootstrap styles on a form to form basis, include the
 ``form_theme`` tag in the templates where those forms are used: