Currently the only way to trigger a notification such as in response to a doorbell ringing is to use the google translate tts function. Unfortunately this doesn’t allow for the same experience as when using a native Google Assistant doorbell.
Looking at the Doorbell device guide Actions on Google Smart Home | Google Developers it looks like the recommended way to achieve this is via the ObjectDetection trait which Home Assistant currently doesn’t appear to implement. I’m not sure if also implementing the doorbell device type is also pre-requisite of this or not but I image there would be value in doing so even if it’s not.
Notifications for smart home Actions | Actions on Google Smart Home also mentions the RunCycle and SensorState traits for other types of notifications (eg, for notifying that a washing machine has finished its cycle, and notifying that a smoke alarm has detected smoke respectively)