Add why_we_use_types.py exercise with bug identification and fix

- Add exercise demonstrating the importance of type hints
- Include buggy double() function that multiplies by 3 instead of 2
- Document the bug and propose two possible fixes

Author: Iswanna

prep/why_we_use_types.py

@@ -0,0 +1,14 @@
+# Question
+
+def double(number):
+    return number * 3
+
+print(double(10))
+
+# Read the above code and write down what the bug is. How would you fix it?
+
+
+# Answer
+# The bug: The function multiplies the number by 3 instead of 2, which contradicts its name.
+
+# Proposed fix: We can either change the function implementation so that it returns number * 2 or we change the function name to 'triple'