These files are processed by example_test.go:TestExamples.
A .golden file is a txtar file with two sections for each example that should be created by doc.Examples from the corresponding .go file.
One section, named EXAMPLE_NAME.Output, contains the example’s output, the value of the field Example.Output.
The other, named EXAMPLE_NAME.Play, contains the formatted code for a playable version of the example, the value of the field Example.Play.
If a section is missing, it is treated as being empty.