feat(mdbook): apply to rust-book.cs.brown.edu

[AxerTheAxe]

🔧 What does this fix? 🔧

Apply the mdbook userstyle to https://rust-book.cs.brown.edu

🗒 Checklist 🗒

• [x] I have read and followed Catppuccin's contributing guidelines.
• [x] I have updated the version appropriately in the ==UserStyle== header of the catppuccin.user.css file.

[isabelroses]

Perhaps we make the url catch all, and we have some application test? But for now I think we merge this and raise an issue for further discussion.

[AxerTheAxe]

Hmm, I'm not sure if I want to add every possible mdBook URL/domain here. I think utilizing Custom included sites (https://github.com/catppuccin/userstyles#individual-userstyles) is probably the way to go here, though I'm curious what @isabelroses thinks. Either way it would probably be worth supporting the https://github.com/cognitive-engineering-lab/mdbook-quiz addon they use, could possibly be used on other sites. I'll open an issue at the upstream https://github.com/catppuccin/mdBook repo.

My rational for this was that if the main version of the book is explicitly included, then it would also make sense to include the secondary version of the book that the main version references and links to. However, I agree that support for MD-book-quiz seems more appropriate here.