docs: sync zero data retention docs from RedPill

[Marvin-Cypher]

Summary

• Add a Phala Confidential AI Zero Data Retention guide based on RedPill's ZDR update
• Document /v1/models?zdr=true, /v1/embeddings/models?zdr=true, and provider: {"zdr": true}
• Clarify that confidential inference (is_tee + receipts) and retention policy (zdr) are separate properties
• Update the RedPill sync script to track the current redpill-ai/redpill-docs source and pin the ZDR commit

Test Plan

• jq empty docs.json
• python3 .scripts/validate-redirects.py
• python3 .scripts/check_links.py
• .scripts/check-redpill-confidential-ai-sync.sh
• git diff --check