Custom Panels example in Tabs component documentation overflows on narrow viewports

The [Custom Panels example](https://primer.style/brand/components/Tabs/#custom-panels) in the Tabs component documentation overflows on narrow viewports.

<img width="299" height="725" alt="Image" src="https://github.com/user-attachments/assets/7ef392ce-377a-43ea-bc71-30d730ad8ccf" />

The component behaves correctly in Storybook, so this may be an issue with our component rendering in Doctocat