* 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
* Docs: improve/expand button documentation
- since we're not doing the disabled cursor thing anymore anyway, remove confusing/unnecessary mention for link-based disabled buttons
- make the disabled toggle example using a link actually follow the preceding advice of also having `tabindex="-1"` and `aria-disabled="true"`
- tweak the link functionality callout to also still mention `aria-disabled` to reinforce the idea
- tweak toggle state description (not just `<button>`s, but also links etc...so just remove mention of `<button>` there; also reinforce accessibility aspect once more)
- add a new callout that cross-references checkbox-based toggles, and what the similarity/difference between them is
- add a matching cross-reference callout to the checkbox-based toggle buttons page
* Update link to now renamed checks-radios.md
* Docs: rename form checks page to "Checks / radios"
it's not immediately obvious that "checks" includes information about both checkboxes and radio buttons. while "checks / radios" is also still not perfect (as honestly even I didn't quite grok that "checks" meant checkboxes), it should make it slightly more obvious nonetheless
