docs: minor story-only accessibility fixes in v9 stories

[smhigley]

This fixes three issues:

1. Popovers with no inner focusables that set focus to the popover surface didn't have an accessible name defined for the surface (this is only necessary if it gets focus, so it's not added for examples that set focus to inner buttons)
2. There was one dialog example that wasn't correctly returning focus
3. Some carousel examples had content get cut off at the top & bottom in high zoom since the height was set with px. Now the min-height is set, which allows the slides to grow if necessary without impacting the appearance at larger widths.

Fixes ADO issues:

• 18661
• 38728
• 38921

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Pull request demo site: URL