Revert: Fix border overlap between Buttons in outlined ButtonGroup

[ggdouglas]

This reverts the changes added in #6966 (while retaining additions to the docs). The removal of borders between Buttons in ButtonGroup had the unintentional consequence of regressing styles for ButtonGroups that contain mixed intents or mixed types of buttons.

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Description

Revert: Fix border overlap between Buttons in outlined ButtonGroup

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[svc-palantir-github]

Keep changes to ButtonGroup docs example

Build artifact links for this commit: documentation | landing | table | demo

This is an automated comment from the deploy-preview CircleCI job.

[evansjohnson]

leaving example of observed undesired behavior

thanks for the fix!