Add numpy example for assigning to a slice

[dstansby]

To assign values to a slice of a numpy array it's possible to do e.g.:

import numpy as np
a = np.zeros((2, 2))
a[:, 0] = 1

It would be good to add how to do this in ndarray to the ndarray for numpy users docs

[jturner314]

How about something like PR #1210?

[nbro]

These examples would really be useful. Why weren't the CI failures addressed yet and the PR merged? Do you need more contributors? I am willing to contribute to this crate, although I am not really an expert in Rust.