docs: add IDC Vendor Interview Platform Technical Guide

[ghost]

Description

• Add comprehensive technical guide for IDC's vendor interview platform built on Vapi
• Document covers architecture (transient assistant pattern), distribution, agent setup, known issues & workarounds, eval & testing, respondent reception, data residency & EU compliance
• Includes code examples for Web SDK integration, tiered silence timeout hooks, speech plan configs, and Deepgram keyword boosting
• Created from technical guidance provided to IDC (contact: Lukas) as part of FDE-157

Linear: VAP5-42

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work
• Verify the markdown renders correctly with all code blocks, tables, and section links

[github-actions]

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