Clarify vapi listen documentation

[goosewin]

Description

• Clarified that vapi listen is a local forwarder only and does not provide an external proxy, tunnel, or public URL.
• Updated documentation across multiple files to explicitly state the requirement for a separate tunneling service (e.g., ngrok) when using vapi listen.
• Removed misleading references to automatic public URLs and webhook-proxy.vapi.ai.
• Revised examples and instructions to include steps for setting up and using tunneling services.

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work, specifically:
  • /cli/webhook-testing
  • /server-url/developing-locally
  • /server-url
  • /cli/overview
  • /debugging
  • /tools/custom-tools
  • /cli/mcp-integration

[github-actions]

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