docs: add simple Go guide

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

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

@@ -0,0 +1,7 @@
+package greeting
+
+import "fmt"
+
+func Greeting(name string) string {
+	return fmt.Sprintf("Hi %s!", name)
+}