chore: remove screenshot from user guide

- Delete the screenshot of MultiPost from the user guide
- Streamline content for better readability and focus on features

Author: harryisfish

content/docs/user-guide/index.mdx

@@ -3,7 +3,6 @@ title: Overview
 description: MultiPost 是一键发布内容到多个社交平台的扩展
 ---
 
-![Screenshot of MultiPost](https://screenshot.domains/multipost.app)
 
 ## MultiPost 功能
 ### 发布