You’re right that the docs are out of date, it’s on the todo-list to update them.
There’ was an older discussion here which may provide some help:
Howver since this uses an Ingress your HTTPS setup is effectively independent of JupyterHub/BinderHub. This means you should be able to follow any other guide to getting HTTPS working with nginx-ingress, and once that’s working configure JupyterHub/BinderHub behind it.