Backend向けWebhookサービスのオンボーディングドキュメント

Finished a documentation-first onboarding template for a backend webhook service.

The goal of this project is to show how to design clear onboarding, architecture docs, ADRs, and runbooks for real backend systems. The code is minimal and serves as a supporting example.

Topics covered include:
— webhook signature verification
— idempotency and duplicate deliveries
— event dispatching
— operational troubleshooting