DOC: add google analytics to the documentation
The main pandas website (pandas.pydata.org) has google analytics tracking, but I think it would be interesting to also have this on the docs, so adding it to the theme.


👍  Who has access to the GA page? Might be good to document that in the wiki. Wes has access, and now gave me access as well on my request. 

BTW, based on the data there, there were apparently about 180 people who clicked on the not-working "pandas-docs/version/0.20.0/" links in the github release message .. :-) 
What is also apparent is that a lot of people still trying to access the docstring page of `pandas.rolling_mean`, which we didn't keep on the website when they were deprecated (probably a bit late now to still add them).