The paper Cadabra: reference guide and tutorial is out of fashion, just 1.x. I wish author could update it to 2.x.
See https://cadabra.science/help.html (not entirely complete but better than nothing).
As you say, it is not entirely complete. We need a more complete reference&tutorial about cadabra2 like other computer algebra systems. If you could update it, we will learn&use cadabra2 more efficiently.
I'm doing my best but there are only 27 hours in each day. Feel free to contribute with tutorials if you think you have learned something that's useful to share.
I am very grateful for your contribution. I just wish it could be finished as soon as possible.