4d: Add the ArticleRepository

Create the `ArticleRepository` class and add it as a service.

This fails as there is no `getAll()` method on the class.

web/modules/custom/my_module/src/Repository/ArticleRepository.php

@@ -0,0 +1,8 @@
+<?php
+
+namespace Drupal\my_module\Repository;
+
+class ArticleRepository {
+
+}
+