Documentation: StaticSiteExport plugin

[jimsafley]

This plugin needs documentation. The "End user notes" in the README is a good starting point. The testing issue may also be helpful.

Right now the ExhibitBuilder, Geolocation, and SimplePages plugins integrate with StaticSiteExport. Before exporting, check out ExhibitBuilder's static-site-export branch, Geolocation's static-site-export branch, and SimplePages' static-site-export branch.