docs: add EU dashboard GDPR compliance guide

[vapi-tasker]

Description

• Adds fern/security-and-privacy/eu-dashboard.mdx — a new page covering the EU dashboard (eu.vapi.ai) and GDPR compliance, directly answering the three most common customer questions: (1) is the EU dashboard GDPR compliant, (2) do I need my own LLM/STT/TTS providers, (3) what else to consider
• Page includes: EU infrastructure overview, provider guidance table (Azure OpenAI EU, Deepgram EU Cloud, etc.), two compliance configurations (provider keys vs full self-hosted), additional GDPR considerations (recording consent, data retention, right to erasure, DPA), US→EU migration checklist, and a summary compliance table
• Registers the new page in fern/docs.yml under the Security and privacy section, after "GDPR compliance", with a fa-light fa-earth-europe icon

Closes VAPSEC-334

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 new "EU dashboard & GDPR" page appears in the Security and privacy sidebar section
• Confirm all internal links resolve (data-flow, GDPR, HIPAA, provider pages, custom LLM/STT/TTS)

[github-actions]

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