chore(button-group): make minor styling adjustments

[dancormier]

[!NOTE] DO NOT MERGE until the temp docs examples added in https://github.com/StackExchange/Stacks/pull/1686/commits/d946ac3b8dd32f63df9808302ae6f84634816ac2 have been removed!

Update Fri Mar 22, 2024: Removed in https://github.com/StackExchange/Stacks/pull/1686/commits/cfbbc199e7ddfd0d989b91d2cf5a63fb151a7b32

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This PR makes the following changes to the button group component requested in the Stacks Classic 2.3.0 update Core PR:

• Adjustment to button padding per sizes (xs, sm, default, md) to match button group height to same-sized standalone button (see related comment)
  • Note: @CGuindon In the original request for this change, you suggested removing the border from these buttons, but I kept it because it's used in the focus state.
  • Note: Since the button component uses custom em values, I didn't exactly match the integer values you suggested @CGuindon. I had to create values like 6.65px to get things to match up.
• Ensure the button child badge doesn't impact button group height (see related comment)
• Remove 1px bottom margin from button group (see related comment)

Check out the deploy preview for examples. I've added some temporary examples of button groups with different sized buttons so we can verify the sizes are what we expect.

[netlify[bot]]

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