update user guide link.

Author: hunghaoti

DoxygenLayout.xml

@@ -37,7 +37,7 @@
     </tab>
     <tab type="examples" visible="yes" title="" intro=""/>
     <tab type="user" visible="yes" title="version log" url="@ref version_log"/>
-    <tab type="user" visible="yes" title="User Guide" url="https://kaihsinwu.gitlab.io/Cytnx_doc"/>
+    <tab type="user" visible="yes" title="User Guide" url="https://cytnx-dev.github.io/Cytnx_doc"/>
   </navindex>
 
   <!-- Layout definition for a class page -->