Add reference link and doctests to is_even algorithm

Author: BhumikaB1

maths/is_even.py

@@ -2,6 +2,14 @@ def is_even(n: int) -> bool:
     """
     Check whether a number is even.
 
+    An even number is an integer divisible by 2.
+    Reference: https://en.wikipedia.org/wiki/Parity_(mathematics)
+
+    >>> is_even(10)
+    True
+    >>> is_even(7)
+    False
+
     :param n: Integer number
     :return: True if even, False otherwise
     """