Replace the word "widget" with "question"

[yanokwa]

We've stopped using "widgets" in favor of "question types" or "question", but the docs are still a bit out of date.

When this change is made, it's important that we keep as many of the anchor links as possible, so for example, if we replaced "text widget" with "text" in the headings at https://docs.getodk.org/form-question-types, the #text-widgets anchor link point to #text