There is no documentation. This is the point of the “inventor”'s kit: you have to invent.
That being said, the idea is that you have access to some of the I/O pins through the I2C sockets or the expansion port. With thee you can make any circuit like on an Arduino. There are tons of tutorials about this everywhere on internet. Here is one related to my first Arduino kit I bought many years ago on Adafruit: http://www.oomlout.com/a/products/ardx/
You will find a couple of simple useful circuits. You may need additional components, but it’s a good start.
I will start writing a coding tutorial soon. I will probably also write something about the inventor’s kit as well.