Add English i18n support with mkdocs-static-i18n

- Add mkdocs-static-i18n to requirements.txt
- Configure i18n plugin: zh (default) at root, en/ subfolder
- Add language switcher (alternate) in extra config
- Disable navigation.instant (incompatible with i18n switcher)
- Add nav_translations for all category labels in English
- Update readme-generate.js: dish/tip nav entries use path-only
  (mkdocs auto-uses page heading = English title for /en/ pages)
- en/ folder to be populated by dotlang translation

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>