docs: add high and medium priority documentation

- Add comprehensive documentation for backend, runtime, and core modules
- Add backend/README.md with architecture, services, and tech stack
- Add runtime/README.md with components and architecture
- Add DEVELOPMENT.md with local development setup guide
- Add ARCHITECTURE.md with system architecture and data flow
- Add backend/api-gateway/README.md with gateway configuration
- Add backend/services/main-application/README.md with module overview
- Add backend/shared/README.md with shared libraries documentation
- Add runtime/datax/README.md with DataX framework guide
- Add runtime/deer-flow/README.md with DeerFlow service guide
- Add runtime/python-executor/README.md with Ray executor guide
- Update README.md with documentation index
- Update README-zh.md with documentation index

Files: backend/README.md, runtime/README.md, DEVELOPMENT.md, ARCHITECTURE.md, backend/api-gateway/README.md, backend/services/main-application/README.md, backend/shared/README.md, runtime/datax/README.md, runtime/deer-flow/README.md, runtime/python-executor/README.md, README.md, README-zh.md

Author: hhhhsc701

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