Replace the word "widget" with "question"

We've stopped using "widgets" in favor of "question types" or "question", but the docs are still a bit out of date.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

Search the documentation for uses of “widget,” starting with the form-question-types page named in the issue. Update the terminology to “question” or “question type,” preserve existing anchor links such as #text-widgets where possible, and verify that the old links still resolve after the headings change.

Written by the indexing model from the issue text.