Add first draft of "Utility methods" page

docs/helpers/utility.md

@@ -0,0 +1,19 @@
+# Utility Methods
+## ensureNotEmpty
+The `ensureNotEmpty` method on the fixture helper checks that any given variable is not [empty](#todo). If it is, it will throw a `LogicException`.
+
+This method also includes the needed annotations, so that [PHPstan](#todo) and [Psalm](#todo) don't throw any errors afterwards:
+
+```php
+<?php
+
+class MyFixture extends Fixture {
+    public function load(): void {
+        $salesChannel = $this->helper->SalesChannel()->getStorefrontSalesChannel();
+        $this->helper->ensureNotEmpty($salesChannel); // [!code focus]
+
+        // Static code analysis now knows that `$salesChannel` exists and is not empty/null. // [!code focus]
+        $salesChannel->getId();
+    }
+}
+```