Update CSS Margin on Hero

If we can remove the `margin: 0 2rem` on mobile strands hero looks fitting better ^^

https://github.com/strands-agents/docs/blob/874fee46cb67a03d69712196dc971e7aa52b1488/src/pages/index.astro#L241

https://github.com/user-attachments/assets/8d3ef57d-22f9-4c6a-b540-3b535b990c5e