Use `grunt doc` in the site-build repo, and then `grunt connect watch` and you'll see the docs at http://localhost:9000/docs/

Those docs are based off of the /built-openseadragon/openseadragon/openseadragon.js in that repo... you can do a `grunt publish` in the openseadragon repo to get your latest over to the site-build repo (assuming you have your folders positioned correctly). Sorry it's not smoother than that. What I usually do is just monkey with the openseadragon.js in site-build to try something out and then make my actual changes back in the openseadragon repo once I've got it figured out.