Add guidance around Undo

Something @matthewpaulthomas [mentioned](https://twitter.com/mpt/status/993367476628017152) is that we're not super clear around how to implement undo actions. We should probably:

- Talk about keyboard shortcuts somewhere (covering Ctrl+Z)
- Update the existing Undo section to mention toasts
- Mention contextual undo/redo actions, like in the context menu in Code

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