As a Universal Package Manager, ProGet offers APIs for all of the different feed types. But there are also API endpoints that help you automate and ProGet itself.
With the exception of the Universal Feed API, the documentation for feed-specific APIs (such as NuGet, npm, etc) is generally found within that product's documentation. We attempt to implement their API as documented and -- more often than not-- as undocumented.
If you have any interest in these or ideas for others, please let us know.
The ProGet Native API is a lower-level API that's designed to automate installation and management of a ProGet instance. It's not terribly intuitive, but its documented here: ProGet Native API Reference