[Cookbook][Web server] description for running PHP's built-in web server in the background

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11311)
| Applies to    | 2.6+
| Fixed tickets | 
>
This starts a web server at ``localhost:8000`` in the background that serves your Symfony application.