Docs examples 05/14: Rework checkbox and radio button examples

Part 5 of 14 in the stack that splits up the approved draft !6295 (closed) into reviewable pieces. The content is exactly what was reviewed there; I've only sliced it.

Related to #3575.

Pages: checkbox, radio button. Realistic content (notification events, merge methods, deploy environments), contextual example placement, unique titles, and five new checkbox Code reference examples showing the options array, child checkboxes with the first slot, custom values with array v-model, contextual states, and an indeterminate select-all.

Safe to publish without the MRs after it.