Document spam call rejection logic

[sahilsuman933]

Description

• Added new documentation page (server-url/spam-call-rejection.mdx) detailing how to reject spam inbound calls using Server URLs and the assistant-request event.
• Included a Node.js/Express example demonstrating how to check the caller's phone number and respond with an error or a transient assistant.
• Updated fern/docs.yml to add the new page to the navigation under "Webhooks".

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work

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[cursor]

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[github-actions]

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