Fix `ButtonGroup` styles for disabled buttons w/ `Tooltip`

Created by: lrhacker

Fixes #40094.

In order for tooltips to work properly on disabled buttons, we need to wrap those buttons with additional elements. This wound up breaking styles when trying to use disabled buttons with tooltips inside of a button group.

This change adjusts the styles for button groups so that they no longer assume the element with the .btn class name will be a direct child. I also moved the ButtonGroup into its own dedicated story and included examples with tooltips to help with testing.

Test plan

Validate visual styles in Storybook. Confirm no visual regressions with ButtonGroups used in the application.

App preview:

• Web
• Storybook

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