Add structured data to generated pages

[mgeisler]

We should look into adding 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: to show the parent pages
• Course: we might be able to distinguish between Rust Fundamentals, Rust in Android, Bare-Metal Rust, and Concurrency in Rust.
• Sitelinks search box: 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.

[mgeisler]

@sakex, a new renderer (https://github.com/google/mdbook-i18n-helpers/issues/70) would probably help with this too.