feature: 优化组件文档

[yushuailiu]

将每个组件的文档重组为以下结构：

1. overview.md - 组件介绍、设计理念、何时使用
2. getting-started.md - 快速入门 + 所有用法示例（合并碎片化页面）
3. api.md - 完整的API参考（属性表、事件表、枚举值说明）

[CLAassistant]

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