Document how instructors can teach from forks

One common pattern of use for lessons was the ability to fork the lesson, modify it, and teach from the fork.

I was given a report of someone who attempted this, but had trouble with GitHub actions. It turns out that GitHub actions does not enable workflows to run by default in forks. The user must

1. enable workflows to run and then,
2. once the workflows create a gh-pages branch, they must direct the pages to build from that branch. (this is [partially explained in the first chapter of the documentation](https://carpentries.github.io/sandpaper-docs/introduction.html#push-to-github), but it could be expanded)