Improve Webhooks Reference: Add Event Table, Testing Guide, and Better Examples

[selvaganesh0355]

• Adding a clear introduction on how Trunk webhooks work and when to use them.
• Including a table of key webhook event types with descriptions.
• Enhancing the "Guides and Examples" section with cleaner links.
• Adding a new section on testing webhooks using tools like Beeceptor and Pipedream RequestBin.
• Ensuring better flow, consistent tone, and GitHub-compatible formatting.