Add analytics to Foundations site

[brian-rose]

As discussed over at ProjectPythia/projectpythia.github.io#114, we have a Project Pythia Google Analytics account to track usage.

We need to add the Foundations book to this tracking.

Should be straightforward:
https://jupyterbook.org/customize/config.html
shows how/where to set the google_analytics_id in our Foundations config.

[brian-rose]

Closing this because we have working analytics now since merging #126. There's a workaround that will be removed after upstream updates as discussed in #128 .