feat: add an example on how to define OpenAPI doc server url (#1644)

Author: kpn13

core/openapi.md

@@ -50,7 +50,7 @@ docker compose exec php \
 Symfony allows to [decorate services](https://symfony.com/doc/current/service_container/service_decoration.html), here we
 need to decorate `api_platform.openapi.factory`.
 
-In the following example, we will see how to override the title of the Swagger documentation and add a custom filter for
+In the following example, we will see how to override the title and the base path URL of the Swagger documentation and add a custom filter for
 the `GET` operation of `/foos` path.
 
 ```yaml
@@ -95,6 +95,9 @@ class OpenApiFactory implements OpenApiFactoryInterface
         $openApi = $openApi->withExtensionProperty('key', 'Custom x-key value');
         $openApi = $openApi->withExtensionProperty('x-value', 'Custom x-value value');
 
+        // to define base path URL
+        $openApi = $openApi->withServers([new Model\Server('https://foo.bar')]);
+
         return $openApi;
     }
 }