Go to repo and go to pages bookmarklets
A couple of years ago I made a gist of a bookmarklet that goes from github pages to the associated repo. I use it on a regular basis.
Going the other direction isn't nearly as common (at least for me) but I realized I did in fact need it so here it is as a gist: go to gh-pages bookmarklet. You're likely to see github's 404 page a lot but I prefer that to skimming a readme or typing and guessing at a github.io url.