docs: update U3-Pro streaming docs for release

[devin-ai-integration]

docs: update U3-Pro streaming docs for release

Summary

Documentation updates ahead of the Universal-3 Pro streaming model release, covering eight changes:

1. Prompting beta warning — Added <Warning title="Prompting: Beta"> to the streaming prompting guide and U3-Pro streaming guide.
2. Parameter rename — Renamed min_end_of_turn_silence_when_confident → min_turn_silence across all docs, API specs, cookbooks, and integration guides (~23 files). Both snake_case and camelCase (minEndOfTurnSilenceWhenConfident → minTurnSilence) variants updated.
3. "Already using AssemblyAI streaming?" tip — Updated to recommend setting min_turn_silence and max_turn_silence when switching to U3-Pro.
4. Native multilingual code switching — Bolded and expanded the callout in the U3-Pro doc, model selection page, and Pipecat U3-Pro guide.
5. "How keyterms prompting works behind the scenes" — Added new section to both the streaming prompting guide and the U3-Pro streaming doc, mirroring the async version.
6. Removed "Public beta" label — Removed the <Warning title="Universal-3-Pro: Public beta"> block from universal-3-pro.mdx (streaming) and prompting-guide.mdx, and removed "Currently in public beta." from the getting-started card in getting-started/universal-3-pro.mdx.
7. Navbar reorder for "Build a Voice Agent" — Unhid and promoted Universal-3 Pro on LiveKit and Universal-3 Pro on Pipecat to the first two positions. Hidden: Best Practices Guide, Livekit intro guide, Pipecat intro guide, Vapi, and voice-agent-best-practices.
8. Streaming voice-agents nav update — In /docs/streaming/voice-agents, replaced the Livekit and Pipecat page entries with links pointing to the corresponding U3-Pro docs (/docs/voice-agents/u3pro-livekit and /docs/voice-agents/u3pro-pipecat). Legacy Livekit, Pipecat, and Vapi pages are now hidden but remain accessible via direct URL.

Updates since last revision

• Streaming voice-agents nav (fern/docs.yml) — In the /docs/streaming/voice-agents section, Livekit and Pipecat are now link entries pointing to the U3-Pro guides instead of page entries. The legacy pages (livekit.mdx, pipecat.mdx, vapi.mdx) are marked hidden: true.

Review & Testing Checklist for Human

• API spec files changed (usm-streaming.yml, streamingApi.yml, __package__.yml, usm-streaming-overrides.yml) — Confirm the backend actually accepts min_turn_silence as the new parameter name. These are breaking spec changes.
• Unintended spec/definition changes — The diff includes additions in __package__.yml (e.g., JsonSchemaConfig type) and large auto-generated changes in assemblyai.yml that appear unrelated to this PR. Verify these are expected and not accidentally pulled in by tooling.
• Navbar link targets — Verify the cross-section links (/docs/voice-agents/u3pro-livekit, /docs/voice-agents/u3pro-pipecat) resolve correctly from the streaming voice-agents section. These link across nav sections.
• Hidden pages still reachable — Confirm that hidden pages (livekit-intro-guide, pipecat-intro-guide, vapi, voice-agent-best-practices, legacy livekit.mdx, pipecat.mdx) remain accessible via direct URL for existing bookmarks.
• Recommended values in "Already using" tip — Verify that min_turn_silence=100 and max_turn_silence=1200 are correct starting-point recommendations for U3-Pro.
• Public beta removal alignment — Verify that removing the "Public beta" label is aligned with product/marketing decision to launch U3-Pro streaming as GA.
• Grep for stale references — Run grep -r "min_end_of_turn_silence_when_confident\|minEndOfTurnSilenceWhenConfident" across the repo to verify all occurrences were renamed.

Test Plan: Preview the docs locally with fern docs dev and manually check:

• "Prompting: Beta" warning still renders correctly in the streaming prompting guide and U3-Pro guide
• "Universal-3-Pro: Public beta" warning is removed from all streaming docs
• Getting-started card no longer says "Currently in public beta"
• All parameter references use the new name consistently
• The "Already using AssemblyAI streaming?" tip displays correctly with the recommended values
• The "How keyterms prompting works behind the scenes" sections match the structure and tone of the async version
• Top-level "Build a Voice Agent" navbar: Shows u3pro-livekit and u3pro-pipecat as first two items. Hidden pages not visible but accessible via direct URL.
• Streaming "Voice agents" subnav: Livekit and Pipecat are links (not pages) that navigate to the U3-Pro guides. Legacy pages hidden but accessible.

Notes

• Requested by: Zackary Klebanoff
• Devin session
• CI status: fern-check ✅, run ✅, lint (merge commit) ✅, lint (branch) ❌ pre-existing issue (JSX comments, not required)

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