Auto merge of #125408 - chriskrycho:chriskrycho/book-updates, r=ehuss

Support mdBook preprocessors for TRPL in rustbook

`rust-lang/book` recently added two mdBook preprocessors. Enable `rustbook` to use those preprocessors for books where they are requested by the `book.toml` by adding the preprocessors as path dependencies, and ignoring them where they are not requested, i.e. by all the books other than TRPL at present.

Addresses rust-lang/book#3927

Author: bors