chore: Add warning for mail-client deprecation (#1958)

* add warning

* own line

* update

Author: deekshas8

docs-js/features/mail-client/mail-client.mdx

@@ -16,6 +16,11 @@ keywords:
   - TypeScript
 ---
 
+:::caution
+This package has been deprecated and is no longer maintained.
+Use at your own risk.
+:::
+
 This documentation describes how you can send e-mails from applications deployed on the SAP BTP to your e-mail servers.
 When defining e-mail servers by using the destination service on the SAP BTP, both `Internet` and `OnPremise` destinations are supported.