feat: update docs

[earayu]

Note

Overhauls document-upload documentation to a comprehensive architecture spec and aligns EN/ZH docs.

• Rewrites docs/design/document_upload_design.md with end-to-end design: two-phase commit model, layered architecture, API endpoints, object store configs (Local/S3), parser chain, format conversion, index types/flows, chunking, DB schemas (document, document_index), state machines, Celery scheduling, performance and error handling
• Mirrors the same content in docs/design/document_upload_design_zh.md
• Replaces frontend-focused web/docs/en-US/design/document_upload_design.md with an architecture summary, quick references (APIs, storage, statuses), parser/index types, and pointers to the main doc

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