DOC: add anonymizeIp for Google analytics in docs
Using a more recent snippet (following their current docs), and adding anonymization of the IP (according to their docs, this should only "slightly reduce the accuracy of geolocation"). 

If we do this, should do the same for main website as well.
lgtm. if anyone @pandas-dev/pandas-core can give this a look see. thanks @jorisvandenbossche 