feat: add documentation for shop endpoint with owned items retrieval

Author: JustYuuto

docs/endpoints/shop.md

@@ -0,0 +1,35 @@
+---
+title: Shop
+sidebar_position: 5
+---
+
+This page contains information about the `/shop` endpoint.
+
+## Get owned items
+
+Get all items that the authenticated user owns.
+
+### Request
+
+```http request
+GET /api/shop/owned
+```
+
+Optional query parameters:
+* `type` - Filter items by type. Possible values: `avatar_decoration`.
+
+### Example Response
+
+```json
+[
+  {
+    "id": "a13bdc64-fd4a-4226-a696-2d5276406ae2",
+    "type": "avatar_decoration",
+    "name": "Crown",
+    "description": "A crown for your avatar.",
+    "price": 5.00, // Price in Euros
+    "expires_at": null // Expiration date, if any
+  }
+  // ...
+]
+```