Document ability to export a tar.gz version of the image

[baronfel]

This was added in https://github.com/dotnet/sdk/pull/35151.

Short version - a new property ContainerArchiveOutputPath is available. If set to a full path, that path is the directory of the generated image. If a relative path is provided, that path is anchored at the current working directory. A file with the name $(ContainerRepository).tar.gz is created at that location containing all of the tags built.