Add code examples for form structure

Author: BurntimeX

docs/php/api/form_builder/structure.md

@@ -115,6 +115,13 @@ The implementing class has to implement the methods `objectType($objectType)`, `
 
 !!! warning "This class should generally not be relied on. Instead, `TemplateFormNode` should be used."
 
+Example:
+
+```php
+CustomFormNode::create('example')
+  ->content('html code')
+```
+
 
 ### `TemplateFormNode`
 
@@ -127,6 +134,13 @@ The implementing class has to implement the methods `objectType($objectType)`, `
   If no template has been set and the node is rendered, an exception will be thrown.
 - `variables(array $variables)` and `getVariables()` can be used to set and get additional variables passed to the template.
 
+Example:
+
+```php
+TemplateFormNode::create('example')
+  ->templateName('example_template_name')
+```
+
 
 ## Form Document
 
@@ -210,6 +224,15 @@ The methods of the interfaces that `FormContainer` is implementing are well docu
 - `appendChild(IFormChildNode $child)`, `appendChildren(array $children)`, and `insertBefore(IFormChildNode $child, $referenceNodeId)` are used to insert new children into the form container.
 - `description($languageItem = null, array $variables = [])` and `label($languageItem = null, array $variables = [])` are used to set the description and the label or title of the form container.
 
+Example:
+
+```php
+$form->appendChild(
+  FormContainer::create('example')
+    ->appendChildren([ ... ])
+  );
+```
+
 
 ## Form Field