Add Terraform migration guide for Unified Phone Experience

[BcnCarlos]

Summary

• Adds new guide: Migrate to Unified Phone Experience with Terraform under Customize > Phone Messages > Unified Phone Experience
• Documents the required two-step terraform apply process for migrating from legacy Guardian phone config to the new tenant-level phone provider
• Covers Twilio and custom Action-based provider configurations
• Adds navigation entry in customize.json

Test plan

• Verify page renders correctly in Mintlify preview
• Confirm navigation link appears under Customize > Phone Messages > Auth0's Unified Phone Experience
• Validate code blocks render with proper HCL syntax highlighting
• Check all internal cross-reference links resolve

🤖 Generated with Claude Code

[mintlify]

Preview deployment for your docs. Learn more about Mintlify Previews.

Project	Status	Preview	Updated (UTC)
auth0	🟢 Ready	View Preview	May 27, 2026, 3:27 PM

💡 Tip: Enable Workflows to automatically generate PRs for you.

[github-actions]

Summary

Status	Count
🔍 Total	12
✅ Successful	7
⏳ Timeouts	0
🔀 Redirected	0
👻 Excluded	4
❓ Unknown	0
🚫 Errors	1
⛔ Unsupported	0

Errors per input

Errors in main/docs/customize/phone-messages/unified-phone/migrate-to-unified-phone-experience-with-terraform.mdx

• [ERROR] https://registry.terraform.io/providers/auth0/auth0/latest/docs/resources/tenant#phone_consolidated_experience-2 | Cannot find fragment: Fragment not found in document. Check if fragment exists or page structure
  Full Github Actions output