Add Penpot library to the downloads

[mayrinck]

Hello guys. A while back I've published this library on Penpot's website. Penpot is an open-source alternative to Figma with a lot of functionalities including importing and exporting files as libraries. Would be nice to have this as one of the download options of the site.

https://penpot.app/penpothub/libraries-templates/open-color