feat: Add documentation for undocumented API endpoints (#8852)

* feat: add documentation for undocumented API endpoints

Creates comprehensive documentation for 8 previously undocumented endpoints:
- Voice Activity Detection (/v1/vad)
- Video Generation (/video)
- Sound Generation (/v1/sound-generation)
- Backend Monitor (/backend/monitor, /backend/shutdown)
- Token Metrics (/tokenMetrics)
- P2P endpoints (/api/p2p/* - 5 sub-endpoints)
- System Info (/system, /version)

Each documentation file includes HTTP method, request/response schemas,
curl examples, sample JSON responses, and error codes.

* docs: remove token-metrics endpoint documentation per review feedback

The token-metrics endpoint is not wired into the HTTP router and
should not be documented per reviewer request.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: move system-info documentation to reference section

Per review feedback, system-info endpoint docs are better suited
for the reference section rather than features.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: localai-bot <localai-bot@noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>