[docs-infra] Demo overflow strategy

[oliviertassinari]

What's the problem? 🤔

Today, the Base UI introduction demo uses a different overflow strategy than the rest of the page. See #39225.

It's relatively frequent for a demo to break the mobile layout, e.g. mui/mui-x#10455 recently. Today, when this happens, it's quite easy to spot, the whole page is broken. So it's great because it's easy to identify. Now, it's also not so great because it breaks the whole docs experience, it's not isolated to one demo that is broken.

What are the requirements? ❓

Some demos need to overflow their container, e.g. https://mui.com/material-ui/react-autocomplete/#combo-box can't have an overflow property today, not until the root problem of #23263 is fixed.

What are our options? 💡

1. No change: overflow: auto like in Base UI introduction demo + no overflow on the other demos
2. No overflow on all the demos
3. overflow: auto everywhere + a warning in the console that flags the demo with a scrollbar + a prop to disable the overflow.