Split guide for public vs. private repos

I've been realizing through [my class](https://github.com/advanced-js/syllabus) that using forks instead of "fake forks" is a huge advantage because then pull requests can be used to submit.  This PR splits the pages to a general overview, with separate ones for public vs. private setup.  Presumably the URLs will match the filenames, but I'll figure that out next.  This PR is also significant in that it steers the guide to be more opinionated, in this case encouraging public repositories.