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/index.html

@@ -112,7 +112,7 @@
                                 href="/?libs=./uncompiled/artplayer-plugin-asr/index.js&example=asr">artplayer-plugin-asr</a>
                         </li>
                         <li>
-                            <a href="/?libs=./uncompiled/artplayer-plugin-pip/index.js&example=pip">artplayer-plugin-pip</a>
+                            <a href="/?libs=./uncompiled/artplayer-plugin-document-pip/index.js&example=document.pip">artplayer-plugin-document-pip</a>
                         </li>
                     </ul>
                     <ul>