Add: Chinese translations for memoryview() and project docs

[shaozk]

What This Changes

• Add Simplified Chinese translation of the memoryview() complexity
  documentation (docs/zh/builtins/memoryview_func.md), bringing the zh
  locale to 14 translated pages.
• Add Simplified Chinese translations of the contributor-facing docs:
  README.zh-CN.md, CONTRIBUTING.zh-CN.md, and TRANSLATING.zh-CN.md.
• Add English / 简体中文 language switchers at the top of both language
  versions of README, CONTRIBUTING, and TRANSLATING.

Why

To make the documentation accessible to Simplified Chinese readers. The zh
locale is a pilot that previously had 13 pages; this adds the built-in
memoryview() page plus the project's onboarding docs, so new Chinese
contributors can get started without switching to English.

Type of Change

• New content
• Bug fix
• Documentation improvement
• Structure/organization

Related Issues

N/A

[heikkitoivonen]

Thanks a lot, this is awesome!

[shaozk]

Thanks a lot, this is awesome!

you are welcome, most of it was done by AI.