Issue #3609157: Update payment docs for the Kessai gateways and point at the Worldline webhook setup

Frank Mablyrequested to merge
issue/yoyaku-3609157:3609157-payment-docs into 1.x

docs/payment.md now describes the gateways as coming from the standalone Kessai engine (manual from Kessai, worldline from kessai_worldline), fixing the stale yoyaku_payment_worldline reference left by the payment refit, and points at the Kessai Worldline README for enabling the server-to-server webhook.

Closes #3609157.

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