Add documentation for certificate expiry alerts. (#942)

* Add documentation for certificate expiry alerts. * Update content/includes/nginx-one/alert-labels.md Co-authored-by: Travis Martin <33876974+travisamartin@users.noreply.github.com> --------- Co-authored-by: Mike Jang <3287976+mjang@users.noreply.github.com> Co-authored-by: Travis Martin <33876974+travisamartin@users.noreply.github.com>

Author: 3 people

content/includes/nginx-one/alert-labels.md

@@ -24,4 +24,6 @@ Go to **Notifications > Alerts**. Select the gear icon and select **Alert Name >
 | NGINXOffline | An NGINX instance is now offline | Major | Verify the host is online. Check the NGINX Agent's status on the instance and ensure it is connected to the NGINX One Console. |
 | NGINXUnavailable | An NGINX instance is now unavailable | Major | Ensure the NGINX Agent and host are active. Verify the NGINX Agent can connect to the NGINX One Console and resolve any network issues. |
 | NewNGINX | A new NGINX instance has connected to NGINX One | Minor | Review the instance details in the NGINX One Console. Confirm availability, CVEs, and recommendations to ensure the instance is operational. |
+| NGINXCertificateExpired | A certificate on an NGINX instance or Config Sync Group has expired | Critical | Get a new certificate from your certificate authority, then update the TLS/SSL certificate in the NGINX One Console. |
+| NGINXCertificateExpiring | A certificate on an NGINX instance or Config Sync Group will expire soon | Major | Renew the certificate through your certificate authority, then update the TLS/SSL certificate in the NGINX One Console. |
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