One thing I will say is that the design document and the Tutorials from ROS 1 get the general idea across. That hasn’t changed in any substantial way in ROS 2, even though the code has. So you can at least get an idea of how it is supposed to work.
But agreed it would be really nice to port those tutorials to ROS 2. It’s on our long term plan to do it (see Tutorials: Write a tf2 intermediate tutorial · Issue #387 · ros2/geometry2 · GitHub), but we just haven’t been able to get to it. As @Katherine_Scott said, any help here would be helpful.