docs: add Vapi Voices legacy migration guide

[vapi-tasker]

Summary

• Add new documentation subpage for Vapi Voices legacy migration at /providers/voice/vapi-voices/legacy-migration
• Document the 9 legacy voices being retired on March 1, 2026: Spencer, Neha, Harry, Cole, Paige, Hana, Lily, Kylie, and Rohan
• Update main Vapi Voices page to remove Rohan from Active Voices section and add it to legacy voices list
• Add link from main page to the new migration guide

Changes

New file: fern/providers/voice/vapi-voices/legacy-migration.mdx

• Warning callout with retirement date
• List of all 9 legacy voices being retired
• Explanation of what this means for users
• Placeholder replacement voice mapping table with all 9 voices and 3 option columns (TBD)
• FAQ section using AccordionGroup with 7 questions covering:
  • How replacement voices were selected
  • What happens after March 1
  • Pricing information
  • How to switch voices
  • Enterprise support options
• Next steps cards linking to active voices and ElevenLabs

Modified: fern/providers/voice/vapi-voices.mdx

• Updated warning to include March 1, 2026 deadline
• Added Rohan to the legacy voices list
• Removed Rohan from Active Voices section
• Added link to the new migration guide

Modified: fern/docs.yml

• Updated navigation to convert Vapi Voices from a page to a section
• Added Legacy migration subpage under Vapi Voices

Test plan

• Verify the new page renders correctly at /providers/voice/vapi-voices/legacy-migration
• Verify navigation shows the new subpage under Vapi Voices
• Verify the Warning, Note, AccordionGroup, and CardGroup components render properly
• Verify the replacement voice table displays correctly
• Verify Rohan is no longer in the Active Voices section on the main page
• Verify the link from the main page to the migration guide works

Related

• Linear Issue: VAP-11571

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