docs: add architecture docs and centralize documentation index; update READMEs and API links

[CJackHwang]

Motivation

• Centralize architecture and module-responsibility information to avoid duplicated/mismatched documentation across README and contributing guides.
• Provide a single source-of-truth for project layout, request flow and module boundaries for developers and contributors.
• Surface the new docs in top-level references so onboarding and deployment docs consistently link to the architecture and API reference.

Description

• Added the architecture reference files docs/ARCHITECTURE.md and docs/ARCHITECTURE.en.md as the developer-facing source of truth for project layout and request flow.
• Added a documentation index docs/README.md and updated multiple topical docs under docs/ including TESTING.md, DEPLOY.md/DEPLOY.en.md, toolcall-semantics.md, and DeepSeek SSE notes to include navigation links to the new architecture and API pages.
• Updated top-level landing files README.MD, README.en.md, API.md, and API.en.md to add cross-links to the new docs and to clarify doc navigation.
• Simplified docs/CONTRIBUTING.md and docs/CONTRIBUTING.en.md by removing the duplicated project-structure block and replacing it with references to the new docs/ARCHITECTURE*.md and doc index.

Testing

• No automated tests were run for this PR because the changes are documentation-only and do not modify executable code.

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Codex Task

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Project	Deployment	Actions	Updated (UTC)
ds2api	Ready	Preview, Comment	Apr 7, 2026 3:51am