update implemented algorithm (#144)

* add implemented algorithms in doc

* update implemented algorithms

docs/implementations.rst

 Implemented Algorithms
 ========================
 
-.. container:: custom-index
 
-    .. raw:: html
+.. toctree::
+   :maxdepth: 1
+   :caption: APIs
 
-        <script type="text/javascript" src='../_static/build_toc_group.js'></script>
+   implementations/pg.rst
+   implementations/dqn.rst
+   implementations/ddpg.rst
+   implementations/ppo.rst
+   implementations/impala.rst
+   implementations/a3c.rst
 
-.. automodule:: parl.algorithms.fluid.impala.impala
-    :members:
-    :no-undoc-members:
-    :show-inheritance:
-
-.. automodule:: parl.algorithms.fluid.a3c
-    :members:
-    :no-undoc-members:
-    :show-inheritance:
-
-.. automodule:: parl.algorithms.fluid.ddpg
-    :members:
-    :no-undoc-members:
-    :show-inheritance:
-
-.. automodule:: parl.algorithms.fluid.dqn
-    :members:
-    :no-undoc-members:
-    :show-inheritance:
-
-.. automodule:: parl.algorithms.fluid.policy_gradient
-    :members:
-    :no-undoc-members:
-    :show-inheritance:
-
-.. automodule:: parl.algorithms.fluid.ppo
-    :members:
-    :no-undoc-members:
-    :show-inheritance:

docs/implementations/a3c.rst

+A3C
+==========
+
+.. automodule:: parl.algorithms.fluid.a3c
+    :members:
+    :no-undoc-members:
+    :show-inheritance:

docs/implementations/ddpg.rst

+DDPG
+==========
+
+.. automodule:: parl.algorithms.fluid.ddpg
+    :members:
+    :no-undoc-members:
+    :show-inheritance:

docs/implementations/dqn.rst

+DQN
+==========
+
+.. automodule:: parl.algorithms.fluid.dqn
+    :members:
+    :no-undoc-members:
+    :show-inheritance:

docs/implementations/impala.rst

+IMPALA
+==========
+
+.. automodule:: parl.algorithms.fluid.impala.impala
+    :members:
+    :no-undoc-members:
+    :show-inheritance:

docs/implementations/pg.rst

+Policy Gradient
+==================
+
+.. automodule:: parl.algorithms.fluid.policy_gradient
+    :members:
+    :no-undoc-members:
+    :show-inheritance:

docs/implementations/ppo.rst

+PPO
+==========
+
+.. automodule:: parl.algorithms.fluid.ppo
+    :members:
+    :no-undoc-members:
+    :show-inheritance: