Add blogroll

I think it would be nice to link to a bunch of different websites that I enjoy, perhaps sorted into different categories.

For now I'll keep the list here:

- https://www.redblobgames.com/blog/
- https://matklad.github.io
- https://blog.rust-lang.org
- https://this-week-in-rust.org

The issue names no file, test, or entry point; first inspect the repository structure to find where blog links belong. Add the listed sites as a categorized blogroll, and verify that the links appear correctly on the site.

Written by the indexing model from the issue text.