input: Write a python function to perform insertion sort on a list of numbers.
  assert insertionSort([3, 8, 15, 2, 6, 10]) == [2, 3, 6, 8, 10, 15]
  assert insertionSort([20, 10, 30, 25, 5]) == [5, 10, 20, 25, 30]
  assert insertionSort([1, 4, 3, 2]) == [1, 2, 3, 4]
output: def insertionSort(arr):
      for i in range(1, len(arr)):
          key = arr[i]
          j = i - 1
          while j >= 0 and key < arr[j]:
              arr[j + 1] = arr[j]
              j -= 1
          arr[j + 1] = key
      return arr