Add an example for optimistic updates with Undo

[elunno]

Description:
Add a React example demonstrating optimistic updates with an Undo pattern:

• User performs add/delete actions
• UI updates optimistically
• Temporary Undo button appears
• If Undo is clicked → rollback
• If not → mutation commits
  A clean and realistic example for teaching optimistic workflows in TanStack Query.

[TkDodo]

not sure I understand this example:

If not → mutation commits

does that mean if I don’t click the button, the mutation never fires? Or does the mutation fire but when you click rollback you’d expect it to abort?