project.json and Visual Studio 2015 with .NET Core
On March 7, 2017, the .NET Core and ASP.NET Core documentation was updated for the release of Visual Studio 2017. The previous version of the documentation used Visual Studio 2015 and pre-release tooling based on the project.json file.
The documentation version from before the March 7 update is available in a PDF file and in a branch in the documentation repository.
The best source of the earlier documentation is the .NET Core - PDF for project.json and Visual Studio 2015.
Documentation repository branch
You can view the earlier version of the documentation in the repository, but many links won't work and many code snippets are references that aren't expanded.
Current version of the documentation