Switch from demo.ragflow.io to cloud.ragflow.io (#13624)

### What problem does this PR solve?

Switch from demo.ragflow.io to cloud.ragflow.io

### Type of change

- [x] Documentation Update

README_ja.md

@@ -1,5 +1,5 @@
 <div align="center">
-<a href="https://demo.ragflow.io/">
+<a href="https://cloud.ragflow.io/">
 <img src="web/src/assets/logo-with-text.svg" width="350" alt="ragflow logo">
 </a>
 </div>
@@ -20,7 +20,7 @@
     <a href="https://x.com/intent/follow?screen_name=infiniflowai" target="_blank">
         <img src="https://img.shields.io/twitter/follow/infiniflow?logo=X&color=%20%23f5f5f5" alt="follow on X(Twitter)">
     </a>
-    <a href="https://demo.ragflow.io" target="_blank">
+    <a href="https://cloud.ragflow.io" target="_blank">
         <img alt="Static Badge" src="https://img.shields.io/badge/Online-Demo-4e6b99">
     </a>
     <a href="https://hub.docker.com/r/infiniflow/ragflow" target="_blank">
@@ -42,7 +42,7 @@
   <a href="https://github.com/infiniflow/ragflow/issues/12241">Roadmap</a> |
   <a href="https://twitter.com/infiniflowai">Twitter</a> |
   <a href="https://discord.gg/NjYzJD3GM3">Discord</a> |
-  <a href="https://demo.ragflow.io">Demo</a>
+  <a href="https://cloud.ragflow.io">Demo</a>
 </h4>
 
 <div align="center" style="margin-top:20px;margin-bottom:20px;">
@@ -59,7 +59,7 @@
 
 ## 🎮 Demo
 
-デモをお試しください：[https://demo.ragflow.io](https://demo.ragflow.io)。
+デモをお試しください：[https://cloud.ragflow.io](https://cloud.ragflow.io)。
 
 <div align="center" style="margin-top:20px;margin-bottom:20px;">
 <img src="https://raw.githubusercontent.com/infiniflow/ragflow-docs/refs/heads/image/image/chunking.gif" width="1200"/>