Document Twilio phone number import with restricted API key

[sahilsuman933]

Description

• Updated Twilio phone number import documentation to recommend and detail the use of Restricted API Keys for enhanced security and least-privilege access.
• Added step-by-step instructions for generating a Twilio Restricted API Key with the necessary permissions.
• Included a cURL example for programmatic import using the Twilio API Key SID and Secret.
• Revised dashboard import steps to align with API Key usage.

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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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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