[test][docs] Improve demos for better regression screenshots

[aarongarciah]

Follow up on https://github.com/mui/material-ui/pull/43656#issuecomment-2348446752

Some demos were using flex instead of inline-flex in their top-level elements, causing them to look off when rendered inside a block element instead of a flex element. For context, our demo wrapper uses flex but our visual regression wrapper uses block since https://github.com/mui/material-ui/pull/43656.