[Task] Update public REST API docs to include the `/reporting/payment_activity` endpoint

[Jinksi]

Part of #8680

In WooPayments 7.9, we will release the Payment Activity Card which consumes the new /reporting/payment_activity endpoint.

We should update the public REST API documentation (https://automattic.github.io/woocommerce-payments/) to detail the changes introduced.

See https://github.com/Automattic/woocommerce-payments/blob/6b5b9a65c9c2701d73c9b0c5f14f640276876cf3/docs/rest-api/README.md for a guide on updating these docs.