Reorganize Chinese translations from docs_cn/ into docs/zh-cn/
following Starlight's i18n content structure. Configure Starlight
with root locale (en, unprefixed) and zh-cn (/zh-cn/ prefix).
- Move 28 files from docs_cn/*_cn.md to docs/zh-cn/*.md
- Fix 21 internal links to remove _cn suffixes
- Add defaultLocale + locales config to astro.config.mjs
- Add .gitignore exception for docs/zh-cn/ (overrides z*/ rule)
- Language switcher activates automatically via existing Header
- hreflang tags auto-generated by Starlight on all pages
Alex Verkhovsky