doc/manual: don't need to copy highlight.js manually

[Pamplemousse]

The theme is totally customizable, you can selectively replace every file from the theme by your own by adding a theme directory next to src folder in your project root. Create a new file with the name of the file you want to override and now that file will be used instead of the default file.

https://rust-lang.github.io/mdBook/format/theme/index.html?highlight=theme%20folder