Add instructions on how to delete Notifications

### Problem

When reading the [Notifications documentation](https://github.com/elementary/docs/blob/master/apis/notifications.md), one cannot find the instructions on how to remove previously sent notifications, only how to replace them.

This might be relevant in certain scenarios, such as, messaging applications. If the user opens the app and the message thread without visiting the notification, then the application can delete the now out-of-context notification.

### Proposal

I would propose one of two options. Either creating a new section for removing old notifications, or add more content to the Replace sections.

For the former, it could be similar in structure where it explains the addition of a Delete button and the code to delete the previous notification.

For the latter, perhaps a information note would suffice (if the code is not extensive) to explain how to "replace the previous notification with nothing".

### Prior Art (Optional)

_No response_