doc: move chainer readme content into sphinx and fix broken link in using_mxnet (#715)

Author: eslesar-aws

doc/index.rst

@@ -95,6 +95,11 @@ Chainer
 *******
 A managed environment for Chainer training and hosting on Amazon SageMaker
 
+.. toctree::
+    :maxdepth: 1
+
+    using_chainer
+
 .. toctree::
     :maxdepth: 2