feat: add weighted average algorithm in maths

[diusazzad]

Describe your change:

This PR introduces the Weighted Average (Weighted Mean) algorithm to the maths directory. While the standard arithmetic mean is already present, a weighted average is essential for scenarios where certain data points contribute more significantly to the final outcome than others (e.g., grading systems, financial portfolio returns).

Robust error handling for empty lists, unequal lengths, and negative weights is included.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests?
• Documentation change?

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, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• 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