Preprocessor to remove yml header for slides in book pages

[nuke-web3]

The raw slides.md file is placed in the page.md file when rendered, although this leads the the yml header of that file to be dumped there too, and it's not useful in that form to the reader. There may be an easy fix to add an mdbook preprocessor to remove that: https://github.com/dvogt23/mdbook-yml-header/tree/main

Considerations

This should work as the slides are rendered first and apart from the book at this time, note that if the reveal tooling is brought into a mdbook preprocessor or something like this, it could be this other yml header remover would remove it before the slides were processsed (and the slides do benefit from it, and need it in cases that custom settings are passed)