docs: Remove unused onAddTask prop and useContext imports from reducer + context example

[danrday]

AddTask no longer needs an onAddTask prop or useContext now that the component reads dispatch directly from context via useTasksDispatch.

• Removes the onAddTask prop from AddTask - the component uses
  useTasksDispatch() directly, so no handler needs to be passed down
• Removes the unused useContext import from both AddTask.js and
  TaskList.js - the custom hooks (useTasksDispatch, useTasks)
  already encapsulate the context calls

No behavior change. This makes the example consistent with the
surrounding explanation, which highlights that context lets child
components skip prop-passing entirely.

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[github-actions]

Size changes

Details

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This PR introduced no changes to the JavaScript bundle! 🙌

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