Docs: separate button group out of checks/radios, expand button groups page

In forms/checks-radios:

• give examples of the pure toggle checks and radios, without the button group class. show that these work fine without .btn-group, but still cross-reference it
• move the explanation from "Checkbox toggle buttons" directly to "Toggle buttons", as the mention of using button classes applies equally to the subsequent "Radio toggle buttons" and "Outlined styles" subsections.

In components/button-groups:

• expand button group page description as it's not just single line, but vertical as well
• add more colour ... .btn-secondary is just dull and uninspiring
• new section to showcase "Checkbox and radio button groups"
• add a mixed styles example
• add an example of vertical radio button group

follow-up from https://github.com/twbs/bootstrap/pull/30650#issuecomment-623126740 which suggested porting some of the docs changes trialled in #28463

Previews: checks & radios toggle buttons / button group