docs: Add data flow documentation for custom storage and models (#878)

* docs: add data flow documentation for custom storage and models

Adds comprehensive documentation explaining data flow when using
custom bucket storage and custom LLM configurations in Vapi.

Includes:
- Mermaid diagrams for default, custom storage, custom LLM, and combined flows
- Data storage summary table
- Use case recommendations
- Cross-references to related documentation

Addresses customer inquiries about what data is passed and stored
in Vapi vs their own systems.

VAP-11220

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

* docs: add data flow page to navigation

Adds the new data flow documentation page to the Security and
privacy section in the docs navigation.

VAP-11220

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

* 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>

* 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>

* docs: simplify BYOK tables to bullet lists

- Convert Speech-to-Text provider table to bullet list format
- Convert Language Model provider table to bullet list format
- Convert Text-to-Speech provider table to bullet list format
- Maintain custom voice references (confirmed supported via CustomVoice class)

VAP-11220

* fix: address PR review feedback

- Fix Custom TTS link to customization/custom-voices/custom-tts
- Update Custom Storage Data Flow: rename "Vapi Metadata" to "Vapi Storage" with Product Usage Metrics and System Logs
- Add clear distinction between System Logs (internal) and Call Logs (customer-facing)
- Ensure consistent naming across all diagrams
- Add Vapi Storage to Custom Models Data Flow diagram
- Fix BYOI Summary: Transport BYOK via Twilio/Telnyx/Vonage, Storage in BYOK column
- Fix Artifacts Storage Summary: Custom Storage as Yes/No, add HIPAA disclaimer

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

---------

Co-authored-by: vapi-tasker[bot] <253425205+vapi-tasker[bot]@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>

Author: vapi-tasker[bot]

fern/docs.yml

@@ -686,6 +686,8 @@ navigation:
             icon: fa-light fa-shield-check
             collapsed: true
             contents:
+              - page: Data flow
+                path: security-and-privacy/data-flow.mdx
               - page: JWT authentication
                 path: customization/jwt-authentication.mdx
               - page: Recording consent plan