[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 you are right about being better to place the `versionadded` directive inside the sidebar.

The problem is that I don't know how to do it. I find Sphinx absurdly complex and the Python/Sphinx community doesn't help us. Take for example the question about adding titles inside the sidebars. We asked for help on GitHub, Twitter and [even on the official Shpinx user group](https://groups.google.com/forum/#!topic/sphinx-users/CovNKsrFY_I). No one helped us.