#6918: docs: add Chinese README

Adds a Chinese README (README.zh-CN.md) and a language switch link in README.md (English | 中文).\n\nThis is a first-pass translation focused on onboarding (install/quick start/security). Longer sections can be iterated on in follow-up PRs. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds a Chinese translation README (`README.zh-CN.md`) and introduces a simple language switch link (`English | 中文`) near the top of `README.md`. The Chinese README mirrors the English README’s initial onboarding/install/security sections, keeping branding/badges and core quick-start commands consistent. <h3>Confidence Score: 4/5</h3> - This PR is safe to merge; changes are docs-only with minor polish opportunities. - The diff only touches README files and doesn’t affect runtime behavior. The main issues are documentation-quality concerns (placeholder/untranslated section list and minor formatting inconsistencies). - README.zh-CN.md <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->