[Cookbook] add cookbook article for the server:run command

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#3465)
| Applies to    | all
| Fixed tickets | 
I like the idea of such an addition. What do you think about a sidebar for this?

However, listening on 127.0.0.1 wouldn't work since it's the loopback device. I guess usually the internal interface is something like 192.168.0.1, 10.0.0.1 or similar.