Rename public to build

[glebec]

Students very often make the mistake of putting things in public, or changing files in public — often to the point of removing public from .gitignore. Part of the problem is we've used public for static routing during the curriculum, but then we switch it up and use public as our build folder for FSG.

We should change the name of this folder to something less tempting — e.g. build. It won't prevent students from messing with it, but it will probably reduce the likelihood at least.