Add doc link for cert renweal error

src/mail_templates/certificate_renewal_error.ejs

@@ -8,6 +8,10 @@ The Cloudron will attempt to renew the certificate every 12 hours
 until the certificate expires (at which point it will switch to
 using the fallback certificate).
 
+See https://cloudron.io/documentation/troubleshooting/#certificates to
+double check if your server is configured correctly to obtain certificates
+via Let's Encrypt.
+
 The error was:
 
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