docs: make README English-first with language switch

[anonymoussoft]

Summary

• make the root README English-first and add a language switch to a new README.zh-CN.md
• keep Chinese documentation paths pointing back to the Chinese README while English entry points stay on the English README
• fix broken relative links in nested documentation pages so local markdown targets resolve correctly

Verification

• git diff --check
• python3 link validation over the updated markdown files (205 local markdown links checked, all targets exist)
• no application test suite was run because this PR only changes documentation