DOC: Update numpy website url
When clicking on the `See Also` item of https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.argsort.html

We go to the old numpy url (docs.scipy.org), and a warning is shown to the user. We should update the target url of our numpy links.

Sphinx builds automatically these links, I guess the url is in `doc/source/conf.py`.

