To my surprise, there is little to no usable information on the topic. Even the page on GitHub "Importing a repository with GitHub Importer", although explicitly mentioning Codeplex, refers to an archived item.
My goal is simple:
I want the source code of my archived C# project, as is, have transferred into a public GitHub repository of mine.
I don't worry about commits, issues, or any history. How to do that in a simple way? Preferrably not involving any download/local processing.