[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 | 
I'm cool with that, but let's make 4 (about the built-in web server) smaller - an example of `server:start` with a link to read more.