Dead links in Mathematics section

[John-Nagle]

Most of the links in the "Mathematics" section of https://rust-lang-nursery.github.io/rust-cookbook/ are dead.

I'd like to suggest adding some library that has the small vectors you need for graphics - 2D, 3D, and 4D, along with the corresponding matrices. Everybody doing graphics needs such a library. I'd suggest "glam", which is used by WGPU, a major graphics library, and Bevy, a game library. It has all the basic, routine operations on small vectors. 3 million downloads.

Then the Rust Playground, which uses the cookbook list, will include them. There are lots of little analytic geometry problems you can't run in the Playground, such as point to line, distance to rectangle, etc.