feat: add Document Picture-in-Picture plugin for ArtPlayer

- Implemented the core functionality of the Document Picture-in-Picture plugin.
- Created main plugin file `artplayer-plugin-document-pip.js` with support for opening and closing the PIP window.
- Added legacy support in `artplayer-plugin-document-pip.legacy.js`.
- Defined plugin options in `package.json` and included necessary metadata.
- Developed the main logic in `src/index.js`, including style copying and event handling.
- Created TypeScript definitions in `types/artplayer-plugin-document-pip.d.ts` for better type safety.

CHANGELOG_CN.md

@@ -9,6 +9,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 -  移除 `svg` 默认的 `fill` 颜色值
 -  优化 `artplayer.d.ts`, 现在可以直接导入 `Option`
 -  优化 `autoOrientation` 逻辑
+-  添加 `artplayer-plugin-document-pip` 插件
 
 ```ts
 import type { Option } from 'artplayer'