docs: clarify EU SIP API host

[dhruva-vapi]

Description

Problem: API users configuring EU SIP trunks could follow the example with api.vapi.ai and create resources in the wrong region.

This docs PR clarifies that EU SIP trunk credentials and BYO phone numbers must be created through https://api.eu.vapi.ai and keeps the API host paired with the matching SIP host.

• Updates fern/advanced/sip/sip-trunk.mdx with a regional API/SIP host note.
• Changes the credential, phone-number, and call examples to use VAPI_API_BASE_URL.
• Converts the outbound call example into a real cURL block so the regional base URL is consistent across examples.

Testing Steps

• Run fern check -- passes with 0 errors.
• Verify JSON examples are valid -- changed page now has 0 JSON blocks.
• Verify internal links resolve -- 1 internal link checked.
• Review page content for style guide compliance.
• Cross-check API host guidance against live EU SIP smoke test behavior.

[dhruva-vapi]

• docs: clarify EU SIP API host #1075 👈 (View in Graphite)
• main

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[github-actions]

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