Add docstrings to ndarray methods

[bryevdv]

This PR adds docstrings to ndarray methods to match the documentation structure of the official Numpy docs:

https://numpy.org/doc/stable/reference/arrays.ndarray.html

A few notes:

• cholesky and convolve do not have direct counterparts, I wanted to solicit guidance on content before authoring any docs for these
• Did not add docstrings for funcs that are NotImplementedError e.g. getfield and setfield

[magnatelee]

cholesky and convolve do not have direct counterparts, I wanted to solicit guidance on content before authoring any docs for these

I don't think we want to expose them to the API. I feel we should rename to _cholesky and _convolve, respectively, to mean that they are internal.

Did not add docstrings for funcs that are NotImplementedError e.g. getfield and setfield

I think you made a right call there. I actually want to remove any method that does nothing more than raising that exception.

[bryevdv]

Merging, since the changes here should not intersect with any other work.