[Doc] Use Markdown syntax highlighting

A clean version of the https://github.com/symfony/symfony/pull/11999

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 
This kind of blocks won't be displayed properly. The intentation should be removed.

    ```php
    use Symfony\Component\EventDispatcher\EventDispatcher;
    use Symfony\Component\EventDispatcher\Event;

    $dispatcher = new EventDispatcher();

    $dispatcher->addListener('event_name', function (Event $event) {
        // ...
    });

    $dispatcher->dispatch('event_name');
    ```

vs

```php
use Symfony\Component\EventDispatcher\EventDispatcher;
use Symfony\Component\EventDispatcher\Event;

$dispatcher = new EventDispatcher();

$dispatcher->addListener('event_name', function (Event $event) {
    // ...
});

$dispatcher->dispatch('event_name');
```