This is the fundamental problem. In order to learn Redux, you need experience with it. In order to get experience, you need to work with Redux. In order to work with Redux, you need to learn it.
You use the example of Rails, but Rails is the poster child of opinionated frameworks. For someone trying to learn it, there's no ambiguity about how a project is structured. The same is true of Unity or the UDK: there's a single tech choice to make, after which you can rely on the framework to guide you.
I understand that the js community prefers small composable libraries, and agree with the reasoning, but with Redux this is taken to extremes: a tiny core library, documentation that's mostly boilerplate, and much greater cognitive overhead than any other state management.