add reference for theta star algorithm

Author: musabkas

docs/modules/5_path_planning/grid_base_search/grid_base_search_main.rst

@@ -97,6 +97,10 @@ As a result, Theta star produces shorter, smoother paths than A star, ideal for
 
 In the animation, each cyan arrow represents a node pointing to its parent.
 
+Reference
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+
+-  `Theta*: Any-Angle Path Planning on Grids <https://cdn.aaai.org/AAAI/2007/AAAI07-187.pdf>`__
 
 Code Link
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