Docs: separate button group out of checks/radios, expand button groups page (#31201)

* Keep checks/radio toggle buttons on topic

- 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.

* Expand button group description

as it's not just single line, but vertical as well

* Expand button group examples

- 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