[FR]: Fix broken CheckBox.md code example

Describe the problem

The standalone Checkbox example in CheckBox.md references isChecked inside onClick, but the declared state variable is checkedState: ToggleableState. The variable isChecked does not exist. The example does not compile.

Describe the solution

Replace the onClick body with correct state mutation:

onClick = { checkedState = if (checkedState == ToggleableState.On) ToggleableState.Off else ToggleableState.On }

Additional context

Priority: High. Effort: XS.