Create branch(es) for built output

[bencomp]

To allow the website to be deployed with git, we need to have the built HTML available via git. This is basically the way that GitHub Pages works, except we want to host the site on our own infrastructure.

Maybe we want to have separate build branches for the develop and (to be created) main/production branches, so that we can review the built website and control when changes go to production.

[bencomp]

Note that the _config.yml defines a site URL; if we create a development and production version, then this URL should be adjusted for the environment.