doc: add go playground demo

pointer/pointer.go

@@ -7,19 +7,19 @@ package pointer
 import "reflect"
 
 // Of returns a pointer to the value `v`.
-// Play: todo
+// Play: https://go.dev/play/p/HFd70x4DrMj
 func Of[T any](v T) *T {
 	return &v
 }
 
 // Unwrap returns the value from the pointer.
-// Play: todo
+// Play: https://go.dev/play/p/cgeu3g7cjWb
 func Unwrap[T any](p *T) T {
 	return *p
 }
 
 // ExtractPointer returns the underlying value by the given interface type
-// Play: todo
+// Play: https://go.dev/play/p/D7HFjeWU2ZP
 func ExtractPointer(value any) any {
 	t := reflect.TypeOf(value)
 	v := reflect.ValueOf(value)