The Swift documentation is here on the Swift.org website. if you were expecting it to be at developer.apple.com, that’d be fair enough, there is plenty there for iOS developers. Swift however, is it’s own Open Source thing, so it gets its own site. I use the epub version of the Swift Programming Language book - its just a bit easier to keep my place.

cs193p.sites.stanford.edu is where I go for the videos and homework assignments for that course.