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.

packages/artplayer-plugin-document-pip/README.md

@@ -0,0 +1,11 @@
+# artplayer-plugin-document-pip
+
+pip plugin for ArtPlayer
+
+## Demo
+
+[https://artplayer.org](https://artplayer.org/?libs=./uncompiled/artplayer-plugin-document-pip/index.js&example=document.pip)
+
+## License
+
+MIT © Harvey Zack