Securely proxy vapi calls with backend config injection

[sahilsuman933]

Description

• Added a new guide (/sdk/web-secure-proxy) detailing how to securely use transient Vapi assistants by injecting server-owned configuration via a proxy, preventing sensitive data from being exposed on the client.
• Included an Express.js example for the proxy implementation and a checklist for security hardening.
• Cross-linked the new guide from the main Web SDK documentation (/sdk/web) and added an informational note in the Web Quickstart (/quickstart/web) for improved discoverability.

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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[github-actions]

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[sahilsuman933]