Add documentation reference for hints and concepts

Added a reference to documentation for hints and concepts related to the problem.

Author: zonedoutdg

Problems/Mathematics/Day-03/q002.md

@@ -70,3 +70,6 @@ In the second sample one can connect two resistors in parallel (obtaining resist
 
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 [PROBLEM LINK](https://codeforces.com/contest/343/problem/A)
+
+refer to the documentation below for hints and concept:
+[ref:](https://en.wikipedia.org/wiki/Calkin%E2%80%93Wilf_tree)