Publish docs automatically

[shawnbot]

Currently our process of running npm run build to update the docs directory (so that we can then publish to GitHub Pages) is a bit tedious and error-prone because:

1. It inevitably causes merge conflicts
2. It's a manual step that I often forget to run before pushing
3. It won't happen automatically when we merge branches

I propose that we build the site on CI (which also ensures that the site always builds) and publish it from there, ideally to a branch-specific URL on the gh-pages branch so that the URL would be, for instance, https://primer.github.io/primer-react/preview/my-branch-name/.

[shawnbot]

Note: I'm marking this as blocked because we have to either start paying for Travis or make this repo public in order to get it building on CI again.

[shawnbot]

This will actually be done via PRs if we decide to merge #226.