form/question anatomy

A section in intro to forms with a screenshot of a question (or a few),
with the various parts labeled (group name, label, hint),
showing the relationship between XLSForm items and rendered form items in Collect.

This is also a good place to put lots of little form related tidbits that don't fit anywhere else, like:

- #376 Form design for Collect hierarchy view: add inner group to control repeat titles (we don't even have anything on question groups outside of repeats at the moment)
- ~#677~

Start by locating the introduction-to-forms documentation and reviewing how existing screenshots and labeled examples are organized. The work is done when it includes a labeled question anatomy example, explains the relationship between XLSForm items and rendered Collect items, and captures the listed form-related tidbits where appropriate.

Written by the indexing model from the issue text.