Add complex number support to linalg.outer

[kgryte]

This PR

• adds complex number support to linalg.outer. No special considerations are required to accommodate complex numbers.
• updates the input and output array data types to be any numeric data type, not just real-valued data types.
• updates guidance to require a numeric data type. If left as "should", would imply that non-numeric data types would be permitted if a library had overriding reasons for doing so. However, for computing the outer product, non-numeric data types are not applicable.

[rgommers]

This is as straightforward as it can get - in it goes