docs: Add docstrings to metric modules missing documentation

[Aman-Cool]

Description

Several metric modules expose public functions without docstrings. This makes it difficult for new contributors to understand:

• what the function computes
• what parameters it expects
• what structure the function returns

Adding clear docstrings would improve maintainability and make the codebase easier to navigate for contributors.

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Affected modules

The following modules contain public functions without docstrings:

• structured_data_metrics/completeness.py
• structured_data_metrics/duplicity.py
• structured_data_metrics/outliers.py
• structured_data_metrics/statistical_rate.py
• structured_data_metrics/representation_rate.py
• structured_data_metrics/add_noise.py
• structured_data_metrics/summary_statistics.py

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Expected fix

Add docstrings to the public functions in these modules following the format described in contributing.rst.

Example format:

def my_metric(data: pd.DataFrame, ...) -> dict:
    """
    One-line summary.

    Parameters
    ----------
    data : pd.DataFrame
        The input dataset.
    ...

    Returns
    -------
    dict
        Description of the returned structure.
    """

Docstrings should clearly explain:

• the purpose of the metric
• expected input parameters
• the structure of the returned result

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Contribution scope

A contributor can choose one or multiple modules, as each module represents a self-contained task.

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Skills needed

• Reading Python code
• Technical writing

No machine learning or AI expertise is required.