docs: reorganize documentation into i18n folder structure

[DayuanJiang]

Summary

• Reorganize documentation into language-specific folders (en/, cn/, ja/)
• Extract Docker section from README.md to separate guide
• Translate all documentation files to Chinese and Japanese
• Fix all relative paths and links across documentation
• Add missing sections to CN/JA READMEs (Desktop Application, Cloudflare deployment)
• Fix broken import statements in cloudflare-deploy guides

Changes

• Created docs/en/, docs/cn/, docs/ja/ folder structure
• Moved existing docs to docs/en/
• Translated ai-providers.md, cloudflare-deploy.md, offline-deployment.md, docker.md to CN/JA
• Updated README.md links to point to new locations
• Synced CN/JA READMEs with EN README content and structure
• Fixed image paths (../../public/ instead of ../public/)
• Fixed MCP Server README links
• Updated port references (6002)
• Fixed provider list to include SGLang and Vercel AI Gateway

Test plan

• Verify all links in README.md work correctly
• Verify all links in CN/JA READMEs work correctly
• Verify image paths render properly on GitHub