[DOCS] Document wadm.notify messages

[brooksmtownsend]

Notifications are used to sync state between different wadm instances, usually to create scalers in response for an app creation event or delete them in response to an app deletion event. We should document what these events payloads are and what they do, perhaps in https://wasmcloud.com/docs/reference/cloud-event-list#rpc-events.

It should be noted that the notify messages should only be used internally.