bug #129 doc: Update "Your first theme" documentation (pocky)

This PR was merged into the 2.4 branch.

Discussion
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Hello!

I am using SyliusThemeBundle on a new Monofony project and after reading documentation (because I always read documentation 👯 ) I seen some problems:

- First about `theme.json`, documentation is always talking about `composer.json` even if it's possible to have a `theme.json` and a `composer.json` (which is explained in docs/theme_configuration_reference.md)
- Second on the ThemeRequestListener which is not accurate for Symfony 5+. `GetResponseEvent` was deprecated in Symfony 4+ for `RequestEvent`

Thanks for reading and hope it will help!

Commits
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5c3824c doc: Update "Your first theme" documentation

docs/your_first_theme.md

@@ -25,7 +25,7 @@ When adding or removing a theme, it's necessary to rebuild the container (same a
 
 ### Theme structure
 
-Themes can override and add both bundle resources and app resources. When your theme configuration is in `SampleTheme/theme.json`,
+Themes can override and add both bundle resources and app resources. When your theme configuration is in `SampleTheme/composer.json`,
 app resources should be located at `SampleTheme/templates` for templates, `SampleTheme/translations` for translations and `SampleTheme/public` for assets.
 To override a specific bundle's template (eg. `FOSUserBundle`), put it in `SampleTheme/templates/bundles/FOSUserBundle` directory.
 
@@ -65,9 +65,13 @@ Create an event listener and register it as listening for `kernel.request` event
 ```php
 use Sylius\Bundle\ThemeBundle\Context\SettableThemeContext;
 use Sylius\Bundle\ThemeBundle\Repository\ThemeRepositoryInterface;
-use Symfony\Component\HttpKernel\Event\GetResponseEvent;
+use Symfony\Component\EventDispatcher\Attribute\AsEventListener;
+use Symfony\Component\HttpKernel\Event\RequestEvent;
 use Symfony\Component\HttpKernel\HttpKernelInterface;
 
+#[AsEventListener(
+    event: RequestEvent::class,
+)]
 final class ThemeRequestListener
 {
     /** @var ThemeRepositoryInterface */
@@ -82,7 +86,7 @@ final class ThemeRequestListener
         $this->themeContext = $themeContext;
     }
 
-    public function onKernelRequest(GetResponseEvent $event): void
+    public function onKernelRequest(RequestEvent $event): void
     {
         if (HttpKernelInterface::MASTER_REQUEST !== $event->getRequestType()) {
             // don't do anything if it's not the master request