Downstream docs for outbox event routing SMTs

Both, the relational and the MongoDB ones.

• https://debezium.io/documentation/reference/stable/transformations/outbox-event-router.html
• https://debezium.io/documentation/reference/stable/transformations/mongodb-outbox-event-router.html (note on incubating status for upstream needs removal)

Docs exist, but may need some tweaking for downstream.