update website instructions to include 4 day workshop schedules

[jennifer-spillane]

The default is still a two-day workshop schedule, which is fine, and I found some instructions on how to change it in this document: https://carpentries.github.io/workshop-template/customization/index.html. But it's still kind of confusing, especially if you're unfamiliar with this kind of code. I figured it out by looking at old workshops again, and stealing their code sections.

I think that providing this link at the top of the section about editing the website files would be good (I think it's linked inside one of the files, but it's easy to miss), and maybe a separate section detailing the things you might need to change specifically if it's an online workshop.