Basic stacked list items container, and flush mode to remove outer borders and rounded corners.

Highlight current selection state using `.active` or lock items with `.disabled` class.

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Generate clean, numbers-prefixed ordered lists using `.list-group-numbered` helper class.

1. Subheading 1

   Content description for item one.
   3
2. Subheading 2

   Content description for item two.
   1
3. Subheading 3

   Content description for item three.
   5

Change layout rows from vertical stack list items to flat horizontal array columns.