docs(voice): add machine detection guide

[Devon-White]

Summary

Adds a platform how-to for detecting a person, answering machine, or fax machine on outbound and inbound calls, following the structure and tone of #660 (outbound calling guide).

• New guide: fern/products/platform/pages/calling/voice/machine-detection.mdx (/docs/platform/voice/machine-detection)
• Covers SWML detect_machine inline in a REST dial, the REST calling.detect command, and Relay detect/detectAnsweringMachine/detectFax in Python and TypeScript
• Examples: leave a message after the beep, hand a live answer to an AI agent, send/receive a fax, detect on a call already in progress, screen inbound calls, and a tuning table
• Themed flow diagram (SVG + llms-only mermaid equivalent)
• Voice overview: new "Detect voicemail and fax machines" card, and a fix to the stale AMD FAQ link (/docs/swml/reference/detect-machine → /docs/swml/reference/calling/detect-machine, "Realtime SDK" → "Server SDKs")
• llms.txt: repoints the "Answering machine detection" bullet to the new guide

Notes for reviewers

• Based on main; links to /docs/platform/voice/outbound-calling resolve once docs: add outbound calling guide #660 merges.
• Every method, parameter, and event field was verified against the shipped signalwire-sdk 3.4.1 and @signalwire/sdk 2.0.5 packages (not just the reference docs), and every SWML builder chain was executed and diffed against the equivalent hand-written JSON/cURL sample.
• Two upstream doc/SDK inconsistencies were worked around rather than fixed: TypeScript Relay has no detectAnsweringMachine()/detectFax() in 2.0.5 (raw detect() used instead, noted in prose), and detect_message_end defaults differ by surface (false in SWML, true in REST calling.detect) — called out in a differences table.
• No changelog entry, no nav yml change (folder-driven nav), no redirects.

Verification

• yarn fern-md-check: all MDX files valid
• All 39 code blocks (cURL, Python, TypeScript, JSON, YAML) extracted and validated: JSON/YAML parse, bash syntax, Python py_compile/pyflakes/mypy, TypeScript tsc --strict against the installed SDK
• All internal links and in-page anchors verified against target slug: frontmatter

Preview: /docs/platform/voice/machine-detection and /docs/platform/voice

[github-actions]

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https://signalwire-preview-docs-693.docs.buildwithfern.com/docs

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