Add link to resource for GFF formatting

[kylegontjes]

Gff files are introduced in class 2. We have a good list of the specific columns in a gff file, but maybe add a link to a good resource that details the information a little more, like:

GFF Info website: https://useast.ensembl.org/info/website/upload/gff.html

Class page: https://github.com/Snitkin-Lab-Umich/EPID582-Winter-2025/tree/main/class/class2