Package a language selector

[mgeisler]

Currently, you have to edit the mdbook theme directly to add a language selector. An example of this can be seen in Comprehensive Rust 🦀.

We should package this up in some way to make it easy for people to apply. This seems non-trivial because the templating system used in mdbook doesn't seem to make it easy to include new blocks of code without editing the main theme.

Some ideas:

• Inject JavaScript into the pages and let this code build the menu client-side.
• Write a tool which can modify the generated HTML to include the menu when mdbook build is called.

[dalance]

Inject JavaScript into the pages and let this code build the menu client-side.

I realized this idea at https://github.com/dalance/translations/blob/master/rust-by-example/theme/language-picker.js .
By this approach, translation can be achieved without modifing the original book only by inserting two files to additional-css and additional-js entry.
This is a part of rust-lang/rust#133562 .