docs: Incomplete documentation of Email service

[lindhe]

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.

[lindhe]

I just realized it's missing another major thing: an example of how to subscribe.

Especially since the service is (without any comment) defined as service.email.gmail, it's very confusing as to how one subscribes to events. Which one of these would be the right annotation to use?

• notifications.argoproj.io/subscribe.<trigger>.email: <recipient>
• notifications.argoproj.io/subscribe.<trigger>.email.gmail: <recipient>
• notifications.argoproj.io/subscribe.<trigger>.gmail: <recipient>

Could I drop the .gmail suffix and just have it be service.email, similar to the examples for Slack?

[lindhe]

Could I drop the .gmail suffix and just have it be service.email, similar to the examples for Slack?

Seems so. ¯\_(ツ)_/¯