MongoDB Atlas: Add username customization docs (#11943)

* MongoDB Atlas: Add username customization docs

* add changelog

* remove changelog; it was added to the relevant go.mod update PR

website/content/api-docs/secret/mongodbatlas.mdx

@@ -25,6 +25,9 @@ In addition to the parameters defined by the Secrets Engines Backend, this plugi
 
 - `public_key` `(string: <required>)` – The Public Programmatic API Key used to authenticate with the MongoDB Atlas API.
 - `private_key` `(string: <required>)` - The Private Programmatic API Key used to connect with MongoDB Atlas API.
+- `username_template` `(string)` - [Template](/docs/concepts/username-templating) describing how
+  dynamic usernames are generated.
+
 
 ### Sample Payload