Order: Kafka Processing from "cart-checkout" to "order-created"

- Update document: topic names as used in code
- The order service should process incoming messages from the "cart-checkout" topic to create a new order.
- The order service should use the payment service to process payment for the order.
- If payment is successful, the order service should mark the order as "paid" and send a message to the "order-created" Kafka topic.