Post an announcement to a Community. Supports text (message), URL-based media (mediaUrl), and broadcast targeting. target='general' (default) posts only to the default announcement subgroup which reaches every community member and returns the message result synchronously. target='all' broadcasts sequentially to every linked subgroup and is ALWAYS dispatched asynchronously — the response contains a reference you can poll via GET /instances/{id}/request/{reference} instead of the message payload. If both message and mediaUrl are provided without mediaCaption, message is used as the media caption. Note: This endpoint is currently in beta and may change without notice.