[TUTORIAL] Introduce frontend folder (#2457)

docs/conf.py

@@ -190,6 +190,7 @@ gallery_dirs = ["tutorials", "vta/tutorials"]
 
 subsection_order = ExplicitOrder(
     ['../tutorials/language',
+     '../tutorials/frontend',
      '../tutorials/optimize',
      '../tutorials/autotvm',
      '../tutorials/dev',

tutorials/frontend/README.txt

+.. _tutorial-frontend:
+
+Compile Deep Learning Models
+----------------------------

tutorials/nnvm/README.txt

 .. _tutorial-nnvm:
 
-Compile Deep Learning Models
-----------------------------
+NNVM Compiler Tutorials
+-----------------------