This project just has simple explanatory examples and illustrative test code, it is not intended to replace the official docs in any way.
The official Kotlin documentation is very good.
https://kotlinlang.org/docs/home.html
The Kotlin By Example section is probably the easiest way to get started with Kotlin.
https://play.kotlinlang.org/byExample/overview
The benefit of the By Example pages is that you can change the code on the page and run it online. Using it as a simple playground for experimentation.
The official 'Basic Syntax' and 'Idioms' pages are very good simple reference guides (once you've used the language):
The Basic Syntax can be a little confusing if you are new to the language because it introduces concepts without any explanation while demonstrating other concepts.