[ENH] Convert all docstrings to numpydoc format

[Copilot]

Converts all docstrings across the pypfopt module from Sphinx/reStructuredText style to NumPy docstring format, with no content changes (content-only fixes are typos/formatting). Fixes #688

Changes

• Format conversion across 13 source files: :param/:type/:return:/:rtype:/:raises → structured Parameters, Returns, Raises sections

Before

def mean_historical_return(prices, returns_data=False, ...):
    """
    :param prices: adjusted closing prices
    :type prices: pd.DataFrame
    :param returns_data: if true, first arg is returns
    :type returns_data: bool, defaults to False.
    :return: annualised mean return for each asset
    :rtype: pd.Series
    """

After

def mean_historical_return(prices, returns_data=False, ...):
    """
    Parameters
    ----------
    prices : pd.DataFrame
        adjusted closing prices
    returns_data : bool, optional
        if true, first arg is returns. Defaults to False.

    Returns
    -------
    pd.Series
        annualised mean return for each asset
    """

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