update docs (#589)

* update docs

* update docs

docs/modules/path_planning/rrt/rrt_star.rst

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+RRT\*
+~~~~~
+
+.. figure:: https://github.com/AtsushiSakai/PythonRoboticsGifs/raw/master/PathPlanning/RRTstar/animation.gif
+
+This is a path planning code with RRT\*
+
+Black circles are obstacles, green line is a searched tree, red crosses are start and goal positions.
+
+Simulation
+^^^^^^^^^^
+
+.. image:: rrt/rrt_star/rrt_star_1_0.png
+   :width: 600px
+
+
+Ref
+^^^
+-  `Sampling-based Algorithms for Optimal Motion Planning <https://arxiv.org/pdf/1105.1186.pdf>`__
+-  `Incremental Sampling-based Algorithms for Optimal Motion Planning <https://arxiv.org/abs/1005.0416>`__
+