Docs examples 06/14: Rework text input and form field examples

Part 6 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: text input, textarea, select, form group, form input group. Realistic content and contextual placement, plus accessibility fixes: the intro examples were bare inputs with no accessible name and are now wrapped in labeled form groups.

Safe to publish without the MRs after it.