What's the problem this feature will solve?

Help standardize on the names people use for project_urls while also helping to get more URLs with nice icons.

Describe the solution you'd like

Document somewhere (maybe https://packaging.python.org/guides/distributing-packages-using-setuptools/#project-urls) the recommended URL labels projects should consider using.

Additional context
Since https://github.com/pypa/warehouse/blob/master/warehouse/templates/packaging/detail.html has special logic to add appropriate icons based on the label provided for these URLs, it might be nice to help guide people towards a set of labels. That way more people get the nice icons and it helps standardize the labels implicitly.