Add support for project_urls, similar to PyPI

[stevepiercy]

I'd like to add an arbitrary link to our package's page on npmjs.com that is highlighted in the right-hand side navigation by adding it to our package's package.json file without adding it to ourREADME.md. Arbitrary URLs could include documentation, sponsorship, and other useful links. [Currently it appears that only homepageandrepository` are supported](https://docs.npmjs.com/creating-a-package-json-file#default-values-extracted-from-the-current-directory).

PyPI offers this feature in either its setup.py or pyproject.toml project file.

[project.urls]
Repository = "https://github.com/plone/plone-sphinx-theme"
Documentation = "https://plone-sphinx-theme.readthedocs.io/"

This renders as shown in the screenshot and following URL.

https://pypi.org/project/plone-sphinx-theme/

Is this something you would consider useful?