docs: add release note for show_collections

Author: vera

doc/release-notes/11558-show-collections.md

@@ -0,0 +1,12 @@
+The Search API now supports a `show_collections` parameter for dataset results.
+When the parameter is set, each result includes a `collections` array showing the dataset’s parent and linked collections. Each entry includes `id`, `name`, and `alias`, for example:
+
+```json
+"collections": [
+  {
+    "id": 11,
+    "name": "My cool collection",
+    "alias": "dvcb50a190"
+  }
+]
+```