A Google search yields the GitHub page for Google Test. I'm finding many links around the web to https://github.com/google/googletest, which now redirects to the GitHub page. The GitHub project has an overview and a readme.md, which mostly explains how to build Google Test.
Where is the official user documentation for Google Test, or some equally useful, comprehensive documentation?
The documentation is in the GitHub repo, as .md files in the googletest/docs directory. Start with Primer.md. And then you can look at more advanced topics.