Add MeshIO to raytracing in an hour example

[felixcremer]

We need the MeshIO package to load the cat data, otherwise I got an error in the first line of the tutorial after loading the packages.

[SimonDanisch]

Oh really? Shouldnt that be part of Makie?
Anyways, I think the doc build should catch this? FileIO should load MeshIO as long as it's reachable in the current Env.

[felixcremer]

The problem then might have been, that I didn't have MeshIO in my current environment because I just installed the packages that are listed at the beginning of the tutorial into a new environment and then ran into a data loading error.

[SimonDanisch]

That could be... I still want to make the tutorial notebooks downloadable, so they can run with the correct env!