docs: Makefile should copy content, not directories
docs: Makefile should copy content, not directories

Before, the Makefile would create `build` and `source` directories instead of
copying the contents of those directories into the /site/ directory for the
docs container.

kklin/kelda,quilt/quilt,kklin/kelda,kayousterhout/quilt,kklin/quilt,kklin/quilt,kayousterhout/quilt,kelda/kelda,NetSys/quilt,LuiseV/quilt,kklin/kelda,ejj/quilt-dev,NetSys/quilt,LuiseV/quilt,kayousterhout/quilt,quilt/quilt,LuiseV/quilt,quilt/quilt,NetSys/quilt,kelda/kelda,kayousterhout/quilt,NetSys/quilt,LuiseV/quilt,kklin/kelda,NetSys/quilt,LuiseV/quilt,kelda/kelda,ejj/quilt-dev,kklin/quilt,quilt/quilt,ejj/quilt-dev,kklin/quilt,ejj/quilt-dev,kelda/kelda