It seems that the godoc tool is not Go modules aware.
A simple godoc -goroot=. serves the project files, but it does not generate documentation for the packages. I tested it from withing the projects source directory, where also the go.mod and go.sum module files are stored.
How to generate documentation for all packages inside a Go module - outside of $GOPATH?
In the release notes of Go 1.12 is written that the godoc tool will not be included in future Go releases and will only be available via go get after Go 1.12. One should use the Go go doc command. However, go doc does not generate "nice to read" HTML pages. Is there an alternative for documentation generation from Go source code which outputs HTML or Markdown?