Add Middleware in the SSL monitoring options list

Middleware is an alternative to Datadog but it's less costly and has better UI. It offers to monitor SSL, uptime etc using it's synthetic monitoring feature.

It is a valuable addition to the list for any tech team looking for a observability platform.

Link - https://middleware.io/product/synthetic-monitoring/

Author: luharshekhar2000-dot

content/en/docs/monitoring-options.md

@@ -18,6 +18,7 @@ There are a number of monitoring options out there, including:
 * [Host-Tracker](https://www.host-tracker.com/)
 * [HeyOnCall](https://heyoncall.com/guides/ssl-certificate-expiration-monitoring) (self-hosted scripts)
 * [CertKit](https://www.certkit.io/)
+* [Middleware](https://middleware.io/product/synthetic-monitoring/)
 
 Please note that all of these services are unaffiliated with ISRG / Let's Encrypt.