docs: add Chinese documentation

[LukeJiaoR]

What & why

Add Chinese translations for the README and contribution guide, with language switch links in both documents. The existing English documentation remains available.

How to test

• git diff --check
• pnpm typecheck, pnpm lint, and package tests were not run because this is a documentation-only change and dependencies were not installed.

AI assistance

• None
• Used AI for: translating and formatting the documentation; reviewed manually.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would change existing behavior)
• Documentation
• Tests only
• Refactor / cleanup

Checklist

• pnpm typecheck passes
• pnpm lint passes
• pnpm format has been run on changed files
• Added or updated tests where it makes sense
• Branch is up to date with main
• PR title follows the area-prefix style for the area being changed (e.g. worker: …, fix(slack): …)
• Read CONTRIBUTING.md (skim counts)

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Summary by cubic

Add Chinese translations for the README and Contributing guide, with language switch links in both English and Chinese docs. This improves accessibility for Chinese-speaking users while keeping the English docs unchanged.

^{Written for commit 2341655. Summary will update on new commits.}

[superlog-app]

Observability review

All clear — no observability concerns found.

This PR understood the assignment.

[LukeJiaoR]

Superseded by #443, which uses the correct branch name.