docstrings/numpy: Support cross-refs for types in "Returns" sections

[pawamoy]

Is your feature request related to a problem? Please describe.
The goal is to have a common set of features between all parsers.

Describe the solution you'd like
The Numpy parser should be able to parse the types in "Returns" section in order to enable automatic cross-references.

from griffe.docstrings.utils import parse_annotation

# annotation is a string
annotation = parse_annotation(annotation, docstring)
# annotation is unchanged, or becomes a Name or Expression! cross-refs enabled