API: Add diagonal and trace to numpy.linalg [Array API]

[mtsokol]

Hi @rgommers @ngoldbaum,

This PR contains another set of small Array API compatibility changes. It adds numpy.linalg.diagonal and numpy.linalg.trace with docs taken from Array API specification.

[charris]

Thanks Mateusz.

[mtsokol]

It looks that I forgot a release note file. I will add it in a follow-up PR.

[mhvk]

Seeing this only because these new additions to the numpy API triggered astropy warnings, and perhaps too late since it is already merged, but wouldn't it be better to tell what the difference is here rather than what is here now? As is, this comment forces the user to either look up the array API or to look at the docstring of np.diagonal to find out that it is the axes that are used. Maybe add something like
"""
This function is Array API compatible, contrary to
:py:func:numpy.diagonal: the matrix is assumed
to be defined by the last two dimensions.
"""

[mtsokol]

You're right - it's clearer about the difference. I will update the docstring.

[mhvk]

Thanks!