chore: update mail client deprecation notes (#1986)

Author: jjtang1985

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

@@ -19,6 +19,9 @@ keywords:
 :::caution
 This package has been deprecated and is no longer maintained.
 Use at your own risk.
+
+Alternatively, you can use the SAP Cloud SDK to fetch a mail destination and send the mail yourself.
+This can be done, for example, by reading the SAP BTP [documentation](https://help.sap.com/docs/connectivity/sap-btp-connectivity-cf/using-tcp-protocol-for-cloud-applications?locale=en-US) or by copying the existing SAP Cloud SDK [source code](https://github.com/SAP/cloud-sdk-js/tree/v3-main/packages/mail-client/src) (just 2 files).
 :::
 
 This documentation describes how you can send e-mails from applications deployed on the SAP BTP to your e-mail servers.