docs(site): fix strict-build doc links

- replace broken mirrored README asset links with GitHub URLs
- keep mkdocs strict build passing after the README refresh

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docs/plugins/actions/flash-card.md

@@ -53,7 +53,7 @@ If this plugin has been useful, a star on [OpenWebUI Extensions](https://github.
 
 ## Preview 📸
 
-![Flash Card Example](flash_card.png)
+![Flash Card Example](https://raw.githubusercontent.com/Fu-Jie/openwebui-extensions/main/plugins/actions/flash-card/flash_card.png)
 
 ## Changelog
 

docs/plugins/actions/flash-card.zh.md

@@ -51,7 +51,7 @@
 
 ## 预览 📸
 
-![闪记卡示例](flash_card_cn.png)
+![闪记卡示例](https://raw.githubusercontent.com/Fu-Jie/openwebui-extensions/main/plugins/actions/flash-card/flash_card_cn.png)
 
 ## 更新日志