Added Count Inversions algorithm (Merge Sort based)

[usman-7455]

Describe your change:

• Add an algorithm

This pull request adds a Python implementation of the Count Inversions algorithm using a modified Merge Sort approach.

Count Inversions is used to determine how far an array is from being sorted by counting the number of out-of-order pairs (i, j) such that i < j and arr[i] > arr[j]. This has applications in ranking systems, bioinformatics, and measuring list similarity.

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, I am not combining unrelated changes.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes. (Note: The filename is Count_Inversion.py; I can rename it to count_inversion.py if needed.)
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.

Wikipedia link: https://en.wikipedia.org/wiki/Inversion_(discrete_mathematics)