add doc for block builder vars

docs/utils/editors/builder.md

@@ -99,6 +99,7 @@ class CustomBlock extends BuilderBlock
             ->setIsContainer(false) // set `true` if the block can contain blocks
             ->setIcon('<i class="fas fa-pencil-alt"></i>')
             ->setTemplate('builder_block/custom.html.twig')
+            ->setClass('col-md-12')
             ->addSetting(name: 'value', label: 'Value', type: 'textarea', attributes: [], default: 'Default value')
         ;
     }
@@ -126,3 +127,28 @@ That's all folks!
 ## Rendering
 
 To render blocks, simply use `{{ value|block_to_html }}`.
+
+If you need to build variables depending of the content, you can override the method `buildVars`:
+
+```php-inline title="src/BuilderBlock/CustomBlock.php"
+namespace App\BuilderBlock;
+
+use App\Core\BuilderBlock\BuilderBlock;
+use Symfony\Component\DependencyInjection\Attribute\AutoconfigureTag;
+
+#[AutoconfigureTag('builder_block.widget')]
+class CustomBlock extends BuilderBlock
+{
+    public function buildVars(array $data)
+    {
+        $this->vars['bar'] = 'bar';
+    }
+}
+```
+
+And you can access variables in the template:
+
+```twig-inline title="templates/builder_block/custom.html.twig"
+{{ vars.bar }}
+```
+