docs: add listmonk and forward email for secure newsletter delivery guide (#2440)

Author: shaunwarman

docs/docs/content/installation.md

@@ -120,7 +120,7 @@ $ helm upgrade \
 <a href="https://zeabur.com/templates/5EDMN6"><img src="https://zeabur.com/button.svg" alt="Deploy on Zeabur" style="max-width: 150px;"/></a>
 
 ## Tutorials
-
+* [Listmonk with Forward Email for Secure Newsletter Delivery](https://forwardemail.net/en/guides/newsletter-with-listmonk)
 * [Informal step-by-step on how to get started with listmonk using *Railway*](https://github.com/knadh/listmonk/issues/120#issuecomment-1421838533)
 * [Step-by-step tutorial for installation and all basic functions. *Amazon EC2, SES, docker & binary*](https://gist.github.com/MaximilianKohler/e5158fcfe6de80a9069926a67afcae11)
 * [Step-by-step guide on how to install and set up listmonk on *AWS Lightsail with docker* (rameerez)](https://github.com/knadh/listmonk/issues/1208)