docs: enhance data flow documentation with detailed pipeline architecture

- Added complete voice pipeline architecture diagram with all components
- Documented all pipeline stages: Transport, STT, Orchestration, LLM, TTS
- Added "bring your own" support matrix for each component
- Clarified orchestration layer is Vapi-proprietary (ephemeral processing)
- Added custom models data flow diagram
- Expanded artifacts storage summary with all artifact types
- Added provider support tables for STT, LLM, and TTS
- Improved use case recommendations

VAP-11220

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

Author: vapi-tasker[bot]