Document `astype` functionality in ndarray

[sgasse]

Hi! I took a stab at Document ndarray's equivalent to NumPy's astype from #597 in this PR:
#1123

Here are a few questions:

• Which other conversion examples would you like to see?
• How can I locally build / access the ndarray_for_numpy_users documentation to see how my changes look like? I cannot find the page when running cargo doc --open.
• Should there be more details on differences in conversion that stem from Rust's type system and are not specific to ndarray? (e.g. np.astype() does not saturate, a.mapv(|x| x as u8 runs a saturated conversion)

[adamreichold]

How can I locally build / access the ndarray_for_numpy_users documentation to see how my changes look like? I cannot find the page when running cargo doc --open.

I think you just need to enable the docs feature, i.e. run

> cargo doc --features docs --open

[sgasse]

Thank you @adamreichold , that worked 🙏