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.

docs/assets/js/common.js

@@ -62,7 +62,7 @@
       './assets/ts/artplayer-plugin-multiple-subtitles.d.ts',
       './assets/ts/artplayer-plugin-vast.d.ts',
       './assets/ts/artplayer-plugin-vtt-thumbnail.d.ts',
-      './assets/ts/artplayer-plugin-pip.d.ts',
+      './assets/ts/artplayer-plugin-document-pip.d.ts',
       './assets/ts/artplayer.d.ts',
     ]