Nightly Build Does Not Push to Github Pages

[mgrover1]

As of now, the nightly-build action does not push the output to github-pages. While this is not neccesarily a bug, pushing to pages (as long as it is building the content) is useful in many cases and it might be useful to add a flag with a default of True.

[brian-rose]

@mgrover1 I implemented a straightforward solution for this in ProjectPythia/cookbook-template#141.

I think we need to put this change in the template rather than in the code in this repo. But we should discuss a bit more whether this is the most sensible default behavior for new cookbooks, and how to document the process by which a cookbook repo can switch between publishing and not publishing the nightly build.