docs: Improve and expand RevenueCat webhooks documentation

[selvaganesh0355]

1. Rewritten sections in a more conversational, developer-focused tone
2. Added guidance on webhook registration, retry behavior, and best practices
3. Included examples for request payloads and webhook testing
4. Introduced external testing tools (Beeceptor and Typed Webhook) for validating implementations
5. Clarified the use of GET /subscribers for syncing subscription status
6. Improved formatting, consistency, and readability throughout the page

[github-actions]

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