Fixes #26202 (closed), closes #32244.
Originally mentioned by me at https://github.com/twbs/bootstrap/pull/32244#issuecomment-760444399.
Uses generated content via CSS to add the numbers inside list group items. This is done because the styling available for normal ordered/numbered lists affords basically no ability to style, place, align, etc the numbers. This way, people can apply their own custom styles to the ::before element and match or differentiate them as desired.