Rust How-to

[optional_preprocessors: add (P1)](https://github.com/john-cd/rust_howto/issues/530)

• mdbook-hide⮳ hides chapters under construction. Install with cargo install mdbook-hide. Uncomment the related section in book.toml. To mark a chapter as hidden, add the following comment anywhere in the Markdown file. It is better to have it at the top of the file for clarity.

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• mdbook-keeper⮳. Install with

cargo install mdbook_keeper --git https://github.com/tfpk/mdbook_keeper.git