Track the number of times the redirect pages are visited

We've setup a few [redirects](https://rust-lang.github.io/mdBook/format/configuration/renderers.html#outputhtmlredirect) in our [`book.toml` file](https://github.com/google/comprehensive-rust/blob/main/book.toml#L44).

I would like to see if it's possible for us to output some Google Analytics code on those pages to track the number of visits to them. The goal would be to delete the redirects after they get no hits in 3 or 6 months.