[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 | 
Let's add a link in case someone just has no idea what a "router" script is - something like:

```sphinx
The built-in web server expects a "router" script (read about
the "router" script at `php.net`_) as an argument.
```