[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 | 
What do you think about this:

```sphinx
This starts a server at `localhost:8000` that executes your Symfony application. The
command will wait and respond to incoming HTTP requests until you terminate it (this
is usually done by pressing Ctrl and C).
```

I'm trying to keep the language a little simpler, but obviously, I want to remain accurate.