[Components][Form] add versionadded for the data collector form extension

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4+
| Fixed tickets | 

As @ahsio pointed out in #3886, the data collector extension was added to the Form component in Symfony 2.4.
@xabbuh this is the code we use to render sidebars:

```python
    def visit_sidebar(self, node):
        self.body.append(self.starttag(node, 'div', CLASS=('admonition-wrapper')))
        self.body.append('<div class="sidebar"></div>')
        self.body.append('<div class="admonition admonition-sidebar">')
        self.set_first_last(node)
        self.in_sidebar = 1

    def depart_sidebar(self, node):
        self.body.append('</div></div>\n')
        self.in_sidebar = None
```