docs: Add a NOTE in mail.md about Gmail SMTP provider

Author: fnonay

docs/alerts/senders/mail.md

@@ -3,6 +3,8 @@
 The mail alert sender emits alert notifications through the `SMTP` protocol.
 !> If you are using a Gmail SMTP provider, you might need to configure [App Passwords](https://support.google.com/accounts/answer/185833?hl=en) for your account
 
+!> If you are using a Gmail SMTP provider, you might need to configure [App Passwords](https://support.google.com/accounts/answer/185833?hl=en) for your account
+
 ### Configuration {docsify-ignore}
 
 The `mail` alert sender configuration is located in the `alertsenders.mail` section.