[ENH] Convert all docstrings to numpydoc format (#712)

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
```python
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
```python
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
    """
```