Add clear reference to webhook scopes documentation in overview

[Copilot]

Fixes #998

The webhook overview page shows how to create a webhook with a scope property (e.g., "scope": "store/order/updated") but didn't provide an obvious way for users to discover all available webhook scopes.

This PR adds a prominent callout immediately after the webhook creation example that directs users to the comprehensive Webhook Events reference where all available scopes are documented.

Before: Users had to scroll through the entire overview page to find references to the webhook events page, and it wasn't clear that's where scopes are documented.

After: Users see a clear, contextual callout right after the webhook creation example that specifically mentions the scope property and points them to the right documentation.

The change is minimal (3 lines) and follows existing documentation patterns with an info callout that provides helpful guidance exactly when users need it.

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