Enhance flexibility by linking book recs to external DB

While not typically recommended due to security issues, linking the book recommendation page to a google sheet could help streamline the code and make development much more easier. Ideally, in the future we could create a React/node framework and host it externally, but this will do for now.

See the [book-rec](https://github.com/physoly/physoly.github.io/tree/book-rec) branch.