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/document/assets/en_start_i18n.md.D4d0sdBl.lean.js

@@ -0,0 +1 @@
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