docs: improve README copy and quick start

[gaomeng1900]

What

Refresh English and Chinese READMEs: clearer slogan, real demo CDN URL, LLM model note, Show and Tell link, and minor copy/layout fixes.

Type

• Breaking change
• Bug fix
• Feature / Improvement
• Refactor / Chores
• Documentation / Website / Demo / Testing

Testing

• Tested in modern browsers
• No console errors
• Types/doc added

Markdown-only change; no automated validation run.

Requirements / 要求

• I have read and follow the Code of Conduct and Contributing Guide . / 我已阅读并遵守行为准则。
• This PR is NOT generated by a bot or AI agent acting autonomously. I have authored or meaningfully reviewed every change. / 此 PR 不是由 bot 或 AI 自主生成的，我已亲自编写或充分审查了每一处变更。