Add structured data to generated pages

We should look into adding [structured data](https://developers.google.com/search/docs/appearance/structured-data/intro-structured-data) to the generated HTML.

I imagine this could be metadata about each page:

- title, though this is probably covered fine with the `title` element,
- when was the page last changed (look this up in Git),
- place in the page hierarchy,

It seems there is special vocabulary available which might be interesting for us:

- [Breadcrumb](https://developers.google.com/search/docs/appearance/structured-data/breadcrumb): to show the parent pages
- [Course](https://developers.google.com/search/docs/appearance/structured-data/course): we might be able to distinguish between [Rust Fundamentals](https://google.github.io/comprehensive-rust/welcome-day-1.html), [Rust in Android](https://google.github.io/comprehensive-rust/android.html), [Bare-Metal Rust](https://google.github.io/comprehensive-rust/bare-metal.html), and [Concurrency in Rust](https://google.github.io/comprehensive-rust/concurrency.html).
- [Sitelinks search box](https://developers.google.com/search/docs/appearance/structured-data/sitelinks-searchbox): we can perhaps get a search box which leads straight to a `mdbook` search.

How to do this? Likely with a new mdbook preprocessor which can inject the data into the publishing pipeline.