[WIP] docs: initial demo for metric panel legend

[Liu-Hong-Wei]

[中文版模板 / Chinese template]

🤔 This is a ...

• [ ] New feature
• [ ] Bug fix
• [ ] TypeScript definition update
• [ ] Bundle size optimization
• [ ] Performance optimization
• [ ] Enhancement feature
• [ ] Refactoring
• [ ] Update dependency
• [ ] Code style optimization
• [ ] Test Case
• [ ] Branch merge
• [ ] Release
• [ ] Site / documentation update
• [X] Demo update
• [ ] Workflow
• [ ] Other (about what?)

🔗 Related issue link

see #2659

🔗 Related PR link

🐞 Bugserver case id

💡 Background and solution

仿照这个示例自定义图例 UI

当图例整体的展示样式不满足需求时，可以通过 visible 属性关闭图例的展示，然后通过提供的相关图例的 api 实现自定义图例的展示及交互。

由此实现自定义图例, 但是代码有些过于冗长, 是不是需要使用 customMark 来编写?

此外, 对于图例作为第二个 HTML 元素, 使用 append 则会遮挡一部分文档

📝 Changelog

Language	Changelog
🇺🇸 English
🇨🇳 Chinese

☑️ Self-Check before Merge

⚠️ Please check all items below before requesting a reviewing. ⚠️

• [x] Doc is updated/provided or not needed
• [x] Demo is updated/provided or not needed
• [x] TypeScript definition is updated/provided or not needed
• [x] Changelog is provided or not needed

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🚀 Summary

copilot:summary

🔍 Walkthrough

copilot:walkthrough