* jupyter/kernels.scm (jupyter-kernel-path, kernel-specs): Export.
* jupyter/guile.scm (guile-kernel-specs, available-kernel-specs-files):
New procedures.
* guix-jupyter-kernel.scm (create-environment): Use
'available-kernel-specs' instead of 'available-kernel-specs-files' and
adjust accordingly.
(start-container)[spawn]: Likewise.
* guix-kernel-demo.ipynb: Add example.