Add 8 new redirects for moved/renamed doc pages

[devin-ai-integration]

Summary

Adds 8 new redirect entries to fern/docs.yml for doc pages that have moved or are being accessed with .md suffixes:

Source	Destination
/docs/migration-guides/universal-to-u3-pro-streaming.md	/docs/streaming/migration-guides/universal-to-u3-pro-streaming
/docs/concurrency	/docs/pre-recorded-audio/concurrency
/docs/migration-guides/gladia_to_aai.md	/docs/streaming/migration-guides/gladia-to-aai-streaming
/docs/voice-agents/u3-pro-partials	/docs/streaming/universal-3-pro/turn-detection-and-partials
/docs/migration-guides/speechmatics_to_aai_streaming.md	/docs/streaming/migration-guides/speechmatics_to_aai_streaming
/docs/guides/apply-llms-to-audio-files	/docs/llm-gateway/apply-llms-to-audio-files
/docs/language-detection.md	/docs/pre-recorded-audio/language-detection
/docs/pipecat-intro-guide.md	/docs/voice-agents/u3pro-pipecat

Several of these are .md-suffixed variants of existing redirects. Others are entirely new paths for pages that moved sections.

Review & Testing Checklist for Human

• Verify each destination URL resolves to a live page (not a 404) after deploy — especially /docs/streaming/universal-3-pro/turn-detection-and-partials and /docs/pre-recorded-audio/concurrency which are new paths without prior redirects
• Confirm the redirect engine correctly handles .md in source paths (e.g. /docs/language-detection.md)
• Spot-check that /docs/pipecat-intro-guide.md → /docs/voice-agents/u3pro-pipecat is the intended destination (the non-.md variant at line 2043 redirects to a different destination: /docs/streaming/pipecat-intro-guide)

Notes

• Recommend testing by visiting each source URL on the preview/staging deploy and confirming the redirect lands on the correct page.

Link to Devin session: https://app.devin.ai/sessions/c49ae7acc1df45ebbf2c8f80fac7eae6
Requested by: @LeeVaughn

[devin-ai-integration]

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