docs(grid): correct import paths, remove controls warning

[tay1orjones]

Closes #9179

Small improvements to the CSS Grid docs in the new @carbon/react storybook.

Changelog

Changed

• fixed import paths
• ensure the controls pane doesn't show warnings

Testing / Reviewing

Read through the Grid docs mdx in the new @carbon/react storybook - make sure it makes sense and is clear to you. Happy to field any questions and improve these in whatever way you think would be helpful!

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[emyarod]

looks good to me