Documentation for webhooks

The sample payloads for the various webhooks don't appear to be up-to-date. For example, the Sent example is using `status` where it should be `event`. The MessageBounced example is missing `event`, `timestamp` etc, as well as the wrapping `payload` key.

Locate the documentation containing the Sent and MessageBounced webhook sample payloads, then compare those examples with the webhook payload structure described in the issue. Update the examples to use the correct event fields and payload wrapper, and verify that the documented samples match the current webhook format.

Written by the indexing model from the issue text.