29165.performance.rst

Rewrite of `np.ndindex` using `itertools.product`
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The `numpy.ndindex` function now uses `itertools.product` internally, 
providing significant improvements in performance for large iteration spaces, 
while maintaining the original behavior and interface.
For example, for an array of shape (50, 60, 90) the NumPy `ndindex` 
benchmark improves performance by a factor 5.2.