docs: add simple Go guide

Signed-off-by: Tanguy ⧓ Herrmann <tanguy@dagger.io>

docs/plans/go-ci/hello/main.go

@@ -0,0 +1,16 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"dagger.io/testgreetci/greeting"
+)
+
+func main() {
+	name := os.Getenv("NAME")
+	if name == "" {
+		name = "John Doe"
+	}
+	fmt.Printf(greeting.Greeting(name))
+}