Add a 'publications' field to package cards

[eliotwrobson]

As discussed, we should add a new publications field to the package cards on the python packages page.

Problem

Currently, our package cards link to JOSS approval for packages that have it. However, some packages have corresponding publications that aren't through JOSS (for example, the PyPartMC package was published in SoftwareX). To encourage submission of these packages, we should allow them to include these publications on their cards.

Proposed Solution

Add a new publications item to the YAML template for each package. This would allow package authors to include a list of publications that are specifically about their package.

The publications field would not be for citations about the algorithms a package implements, but rather for publications that are about the package itself.

This change would:

Help us to better represent the breadth of packages in our ecosystem.

Make it easier for reviewers and EiCs to understand the scope of a package.

Encourage the submission of packages with existing publications.