Replace "Submit" button with prominent link to new Contributors Guide

[brian-rose]

As discussed in ProjectPythia/projectpythia.github.io#425, we are moving the Cookbook Contributors guide to a more prominent location on our main site.

Along with that, we are putting the "Submit a new Cookbook" link / button in the guide itself. Instead of that button appearing on the gallery page, we should have a prominent button link to the Contributors Guide

[brian-rose]

Closed by #203