[collapsible] Add `unmountOnExit` prop

[mj12albert]

Follow up to https://github.com/mui/base-ui/pull/577#pullrequestreview-2345745521

Do we have an API to not mount the children of an accordion? Something similar to https://mui.com/material-ui/react-accordion/#performance. When rendering heavy components under an accordion, e.g. in a dashboard: charts, data grid, etc. It can be a lifesaver for the performance responsiveness on the page.

Or is the idea to solve this problem with a new component of this kind: https://ark-ui.com/react/docs/utilities/presence?