docs: Incomplete documentation of Email service

Currently, the documentation for the Email service does not have an example of how to configure credentials in the `argocd-notifications-secret` Secret object. Adding that would be a huge benefit for the reader.

Locate the documentation section for the Email service and review how other services document Secret configuration. Add an example showing credentials in the argocd-notifications-secret Secret object; the work is done when readers can see the required configuration clearly in the Email service documentation.

Written by the indexing model from the issue text.