docs: update webhook event documentation

[mkurapov]

Changes proposed in this pull request

After starting work on issue

• 2465

we realized that instead of adding custom retry behavior for any of our webhook requets, we should keep the simple retry mechanism of "retry as long as we don't get back a 200 response", and update the documentation for webhook events in general.

This change:

• Renames "Event Handlers" page to "Webhook Events"
• Moves over "event types" table from the "Getting Started" page to the "Webhook Events" page
• Adds additional sections to the "Webhook Events" page: Setup, Errors, and Best Practices
• Adds sample bruno requests for the webhook listener, to give integrators an easy way to see the webhook payload and test the integration manually

Context

Fixes #2761

Checklist

• [x] Related issues linked using fixes #number
• [ ] Tests added/updated
• [x] Documentation added
• [ ] Make sure that all checks pass
• [x] Bruno collection updated

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