Migrating Physical Oceanography content to Pythia Cookbooks

Originally posted in the older sphinx-gallery (see the description and full conversation there): https://github.com/ProjectPythia/cookbook-gallery-sphinx/issues/23:

> One of the Pangeo Gallery items listed on ProjectPythiaCookbooks/projectpythiacookbooks.github.io#20 is http://gallery.pangeo.io/repos/pangeo-gallery/physical-oceanography/