Docs: Add dashboards previews alias (#109358)

Co-authored-by: Jack Baldry <jack.baldry@grafana.com>

docs/sources/dashboards/_index.md

@@ -1,6 +1,7 @@
 ---
 aliases:
-  - features/dashboard/dashboards/
+  - features/dashboard/dashboards/ # /docs/grafana/latest/features/dashboard/dashboards/
+  - dashboards/previews/ # /docs/grafana/latest/dashboards/previews/
 labels:
   products:
     - cloud