docs: add SIP trunk credential validation troubleshooting guide

[vapi-tasker]

Description

• Add new troubleshooting page fern/advanced/sip/troubleshoot-sip-trunk-credential-errors.mdx for the "Couldn't validate SIP trunk credential. SIP gateway creation failed." error
• Register the page in fern/docs.yml under the SIP integration section, after the Providers sub-section
• Covers the three most common causes of this error:
  1. Using a hostname instead of an IPv4 address in gateways[].ip
  2. Having inboundEnabled defaulting to true on outbound-only trunks
  3. Missing Vapi SBC IP addresses on the carrier's allowlist
• Includes a full gateway configuration reference table and escalation guidance
• Content was CSE-approved (v2 draft) and converted to Fern MDX format with proper components (<Steps>, <Note>, <Warning>, <Tip>, tables)

Resolves: VAPICS-161

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 page appears in the SIP integration sidebar navigation after the Providers section
• Verify the slug advanced/sip/troubleshoot-sip-trunk-credential-errors resolves correctly
• Confirm all internal links in the Next steps section work

[vapi-tasker]

CI Check Analysis

The preview-go check is failing, but this is a pre-existing issue unrelated to this PR:

• The same preview-go failure occurs on PR escape brackets #924 (already merged to main)
• The errors are Go syntax errors in Fern-generated SDK files (assistants.go, calls.go, types.go) — not in any documentation files
• The fern-check (docs validation) passes ✅
• This PR only adds a new .mdx file and a 2-line navigation entry in docs.yml

No action needed — this failure is in the Go SDK code generator and needs to be fixed upstream in the Fern config, not in this docs-only PR.

[github-actions]

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