Web links for links-preprocessor

[Urhengulas]

Hi 👋🏾 ,
I'd like to be able to include a markdown file from another repository into my mdbook. This feature exists for relative links, but no web links yet. I imagine similar to this:

# relative link
{{#include file.rs}}

# web link
{{#include https://raw.githubusercontent.com/user/repo/main/MARKDOWN.md}}

Since this will add some dependencies to fetch the resource, it probably should be hidden behind some config option in book.toml and not be enabled by default.

I am happy to do the changes :D