docs: Preview label for Cube Cloud for Sheets

Author: igorlukanin

docs/pages/product/apis-integrations/google-sheets.mdx

@@ -6,10 +6,16 @@ Cube Cloud.
 <SuccessBox>
 
 Cube Cloud for Sheets is available in Cube Cloud on [Premium and
-above](https://cube.dev/pricing) plans.
+above](https://cube.dev/pricing) plans. It's currently in preview.
 
 </SuccessBox>
 
+<WarningBox>
+
+Cube Cloud for Sheets is currently in preview.
+
+</WarningBox>
+
 After [installing](#installation) and [authenticating](#authentication) this
 add-on, you will be able to [create reports via pivot
 table](#create-reports-via-pivot-table) and work with [saved