Replace md to rst for doc index

doc/index.md

-PaddlePaddle Documentation
-==========================
-
-User Guide
-----------
-* [Introduction](introduction/index.md)
-* [Quick Start](demo/quick_start/index_en.md)
-* [Build and Installation](build/index.rst)
-* [Contribute Code](build/contribute_to_paddle.md)
-* [User Interface](ui/index.md)
-* [Model Config Interface](ui/api/trainer_config_helpers/index.md)
-* [Example and Demo](demo/index.md)
-* [Cluster Train](cluster/index.md)
-
-Development Guide
------------------
-* [Layer Documents](layer.md)
-* [Writing New Layers](dev/new_layer/index.rst)
-* [Source Code Documents](source/index.md)
-* [GPU Profiling Documents](optimization/index.rst)
-
-Algorithm Tutorial
-------------------
-* [RNN Configuration](algorithm/rnn/rnn.rst)

doc/index.rst

@@ -8,3 +8,4 @@ PaddlePaddle Documentation
   user_guide.rst
   dev/index.rst
   algorithm/index.rst
+  optimization/index.rst