F5-Sites: Add item for community forum (#1100)

Author: TasoOneAsia

data/f5-sites.yaml

@@ -13,3 +13,7 @@
 - title: NGINX
   url: https://nginx.org/
   description: Learn more about NGINX Open Source and read the community blog
+
+- title: Community Forum
+  url: https://community.nginx.org/
+  description: Engage with the broader NGINX community for discussions and troubleshooting