add website preview in pull requests

[scottyhq]

project pythia is currently working on a jupyter book that uses read-the-docs to 'preview' website changes before they get merged
ProjectPythia/projectpythia.github.io#17

our CI system is currently a little different. we run the build through docker so that we guarantee to use the exact same environment and package versions that are used on the hackweek jupyterhub.

i think we could use netlify to do website previews https://jupyterbook.org/publish/netlify.html