[docs] Add more demos

# Docs feedback

## How can we improve Base UI documentation?

Add more demos on the components to make it easier to showcase different use-cases.

- dialog
- [x] dialog opens imperatively (we have a code-block)
- [ ] move focus to another element after closing a dialog
- [ ] reset dialog state on open/close
- popover
- [x] open on hover
- menu
- [x] menu checkbox items
- [x] menu radio items
- accordion
- [x] single
- [x] hidden until found
- collapsable
- [x] hidden until found
- [ ] initially open collapsible
- [ ] semi open state (e.g. codeblock that shows only first few lines)
- toggle
- [x] multiple https://github.com/mui/base-ui/pull/4343
- tooltip
- [ ] trigger with custom elements?
- scroll area
- [ ] hard scrollbar, stable gutter, etc
- [ ] vertical & horizontal scrollbar
- [ ] overflow behaviour
- slider
- [x] vertical https://github.com/mui/base-ui/pull/4346
- [ ] steps
- field
- [ ] field custom error
- [ ] field custom element
- [ ] input/checkbox/etc in field
- checkbox / checkbox group
- [x] indeterminate checkbox https://github.com/mui/base-ui/pull/4345
- [x] parent checkbox
- tabs
- [x] tabs as links https://github.com/mui/base-ui/pull/4341
- [ ] scrollable tabs?
- [ ] animated tabs panels
- other
- [ ] form with more validation examples
- [ ] hide when detached
- [ ] custom positioner