The official documentation is hosted on ReadTheDocs. It is maintained by the Godot community in its own GitHub repository.
The class reference is also accessible from the Godot editor.
The official demos are maintained in their own GitHub repository as well.
There are also a number of other learning resources provided by the community, such as text and video tutorials, demos, etc. Consult the community channels for more information.
See this page for a list of supported Godot versions.