Docs examples 07/14: Rework form structure and date picker examples

Part 7 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: multi-step form template, form fields, date picker, password input. Realistic content and contextual placement. Date picker also loses one duplicate GlFormDate example; the surviving "Native date input" example is linked from the accessibility prose instead.

Safe to publish without the MRs after it.