Workflow simplification source

This is the first of 2 coupled PRs.  The purpose is to streamline the workflow for updating the site by building the preview in the source branch directly.  The matching PR in the master branch is not critical.

Testing of the final `gh-pages` capability is delicate since it, by definition, must touch the live site.

Perhaps @scopatz can review and we can propagate this workflow through to other sites/repos/projects.