Add ReplicaSet configuration notice to MongoDB documentation

Author: Julie POUNY

content/doc/addons/mongodb.md

@@ -62,6 +62,14 @@ Encryption at rest is available on MongoDB. You can have more information on the
 
 {{% content "db-migration" %}}
 
+## ReplicaSet Configuration
+
+{{< callout type="info" >}}
+**Need a ReplicaSet setup?** Clever Cloud supports switching your MongoDB add-on to a ReplicaSet configuration (single-node) for features like change streams, transactions, or oplog access.
+
+To discuss your requirements and enable ReplicaSet on your add-on, please [contact our support team](https://console.clever-cloud.com/ticket-center-choice).
+{{< /callout >}}
+
 ## Can I use Mongo Ops Manager on Clever Cloud?
 
 To be able to use [Mongo Ops Manager](https://www.mongodb.com/products/ops-manager), you'll need a valid MongoDB Enterprise Advanced subscription and to deploy a [Linux version of their manager solution](https://www.mongodb.com/try/download/ops-manager). If you haven't purchased any license from MongoDB and you are using the Community version, you might be looking for a similar service for your databases.