The official documentation is severly lacking. Right now, I merely use it for specific API details. As Milana says, the documentation is hostile to those of us coming from a PHP background. Also, the examples shown there are pretty basic and mostly useless for real-world development. Thanks to asking in StackOverflow and following tutorials in some developer blogs I managed to begin to grasp how to navigate this new environment. Also, most of the official documentation is centered in building blocks. If you need to adapt an old plugin that registers a custom post type and some meta boxes to the new Gutenberg UI, you are out of luck finding any decent tutorial there.