docs: apply feedback to data flow documentation

Changes based on feedback:
1. Simplified "Complete Pipeline Flow" diagram - removed inner boxes from
   STT, LLM sections, removed Post-Processing
2. Updated STT table with correct BYOK support based on credential.types.ts
   (Deepgram, Gladia, AssemblyAI, Speechmatics, Google, Azure all support BYOK)
3. Removed "Custom Server" column from component tables, added notes with
   links to custom integration docs instead
4. Renamed "Custom API Keys" to "Bring your own API Key" throughout
5. Ensured consistent naming across all diagrams (Transport, Transcriber,
   Orchestration, LLM, Voice)
6. Fixed custom models diagram layout - user at top, custom servers at bottom

VAP-11220

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

Author: vapi-tasker[bot]