FluidModel Fix (#611)

- delete the Models index in root index
- update models/index_en.rst

doc/fluid/index_en.rst

@@ -11,5 +11,4 @@
   dev/index_en.rst
   api/index_en.rst
   book/index_en.rst
-  user_guides/models/index_en.rst
   advanced_usage/deploy/index_mobile.rst

doc/fluid/user_guides/index_en.rst

@@ -30,4 +30,4 @@ Reproduced classic models of multiple directions in Fluid：
     howto/configure_simple_model/index_en.rst
     howto/training/index_en.rst
     howto/evaluation_and_debugging/index_en.rst
-    models/model12_en.rst
+    models/index_en.rst