Add Google Analytics to the MdBook GitHub Pages

[markmandel]

Is your feature request related to a problem? Please describe.

We should have Google Analytics on our mdbook pages so we can see where stuff is coming from!

Describe the solution you'd like

https://analytics.google.com/analytics/web/

Describe alternatives you've considered

Just use GitHub Analytics, but that is very limited.

Additional context

Can't think of anything else.