Move all the md files to the doc && Update pubspec.yaml of the extensions package

README.md

@@ -741,8 +741,8 @@ Made with [contrib.rocks](https://contrib.rocks).
 [Youtube Playlist]: https://youtube.com/playlist?list=PLbhaS_83B97vONkOAWGJrSXWX58et9zZ2
 [Slack Group]: https://join.slack.com/t/bulletjournal1024/shared_invite/zt-fys7t9hi-ITVU5PGDen1rNRyCjdcQ2g
 [Sample Page]: https://github.com/singerdmx/flutter-quill/blob/master/example/lib/pages/home_page.dart
-[Code Introduction]: https://github.com/singerdmx/flutter-quill/blob/master/CodeIntroduction.md
+[Code Introduction]: https://github.com/singerdmx/flutter-quill/blob/master/doc/CodeIntroduction.md
 
 <hr/>
 
-[中文文档](./doc_cn.md)
+[中文文档](./doc/readme/cn.md)