[Docs] Write a webhook integration guide for external subscribers

[Akpolo]

https://t.me/+DOylgFv1jyJlNzM0

Why this matters

RemitLend has a full webhook system — subscription CRUD (#236/#820), retry with backoff and a circuit breaker (#549/#655), and (incoming) HMAC signing of deliveries (#880) — but there is no single document an external integrator can follow to consume it. They have to read source to learn the supported event types, the payload shape, how to verify the HMAC signature, and the retry/delivery semantics.

Acceptance criteria

• New doc (e.g. docs/webhooks.md) covering: how to create a subscription, the full list of supported event types, an example payload per event, and delivery/retry semantics
• Document the X-RemitLend-Signature HMAC-SHA256 header and a verification snippet (depends on / cross-links [Backend] Sign outgoing webhook deliveries with HMAC-SHA256 X-RemitLend-Signature header #880)
• Document expected subscriber response codes and the circuit-breaker behavior
• Link the guide from the README

Files to touch

• docs/webhooks.md (new)
• README.md

Out of scope

• Implementing HMAC signing ([Backend] Sign outgoing webhook deliveries with HMAC-SHA256 X-RemitLend-Signature header #880)
• Building a subscriber sample app

[ExcelDsigN-tech]

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