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)
+}

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

@@ -0,0 +1,13 @@
+package greeting
+
+import "testing"
+
+func TestGreeting(t *testing.T) {
+	name := "Dagger Test"
+	expect := "Hi Dagger Test!"
+	value := Greeting(name)
+
+	if expect != value {
+		t.Fatalf("Hello(%s) = '%s', expected '%s'", name, value, expect)
+	}
+}