Clarify cookbook repository access issues in the Cookbook Guide

[brian-rose]

Based on this discourse thread, it seems like the steps to generate a Zenodo DOI in our Cookbook Guide require that the author has admin-level access to the cookbook repository.

This is probably a good idea anyway for other maintenance needs.

We should clarify in the Guide that the author should request admin access. Anyone who is an "owner" of the ProjectPythia organization should be able to grant this.

[jukent]

Now mentioned in #536