[docs] add zh docs (#3507)
* [docs] add zh docs
* [docs] add lang dropdown
* [docs] update mkdocs zh config
* [docs] migrate assets
* [docs] update overrides dir in mkdocs zh config
* [docs] exclude locales director in main mkdocs config
* [docs] rename assets to public to avoid conflicting with template
* [docs] extra_css change followup
* [docs] add theme.palette.toggle.icon back into mkdocs zh config
* [docs] fix zh readme reference + migrate language-specific repo markdown to docs
* [docs] translate remaining repo docs + update reference
* [docs] update zh index.md reference
* [docs/zh] wording alignment
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