docs: add number warming guide for outbound campaigns

[justincrowe-hub]

Description

• Adds Outbound Campaigns > Advanced > Warm outbound numbers (/outbound-campaigns/number-warming). It covers warming production numbers with small recurring campaigns to approved contacts who expect the call.
• Defines a custom call-outcome structured output (live_success, live_incomplete, voicemail, screening_only, other_automated, unknown, plus a screeningEncountered flag). The page includes a ready-to-POST schema and shows how to attach it through artifactPlan.structuredOutputIds.
• Covers the gap that structured outputs don't appear on the campaign page. A TypeScript example lists a campaign's contacts (GET /v2/campaign/{id}/contacts), fetches each dialed call (GET /call/{id}), and reads artifact.structuredOutputs. It falls back to endedReason and counts how many calls actually counted toward warming.
• Defines the per-number report and completed conversation rate (live_success / calls actually dialed). The page explains how this rate differs from the campaign Pick up rate.
• Adds reputation checks: Hiya's free Number Registration and Caller Reputation, and Twilio Trust & Engagement Insights, which requires Voice Insights Advanced Features. It notes there is no universal warming score.
• Adds the page to docs.yml navigation.

API fields and endpoints were checked against fern/apis/api/openapi.json: Call.campaignId, CampaignContactWithOutcome.callId, contacts pagination, and the endedReason values used. Hiya and Twilio details were checked against vendor pages on 2026-10-15.

Testing Steps

• node scripts/fern/run.cjs check with the pinned Fern 5.151.1: 0 errors and 14 warnings, the same as main
• Open the preview deployment and confirm the page renders under Outbound Campaigns > Advanced
• Confirm internal links resolve (#trusted-calling-and-caller-id, #receive-campaign-events, #how-campaigns-work, contacts API reference)
• Run the aggregation snippet against a test campaign

🤖 Generated with Claude Code

[lightsage-app]

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Docs URL: https://vapi-preview-01a13fd3-0a8e-747b-8e56-8258b3b4fb02.docs.buildwithfern.com
Commit: 0e6a49c
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[github-actions]

🌿 Preview your docs: https://vapi-preview-01a13fd3-0a8e-747b-8e56-8258b3b4fb02.docs.buildwithfern.com

[justincrowe-hub]

@dhruva-reddy @dhruva-vapi should review this before it's approved