Add sample introduction screens for background recording

Follow on from #1327:

- [ ] Examples of introduction screens. I'd like to do one that's just informational, one that asks for consent and one that allows the respondent to pick topics they're comfortable discussing while being recorded. I'm hesitating between attaching XLSX files and putting the form defs inline but collapsed by default. I'd love thoughts on that or other options!

CC @seadowg for anything else we should improve with https://docs.getodk.org/form-question-types/#background-audio-recording

Start with the background audio recording documentation at docs.getodk.org/form-question-types/#background-audio-recording and review follow-on issue #1327. Decide how the examples should be presented, then add one informational introduction screen, one consent screen, and one topic-selection screen. Done means all three sample screens are documented in a format agreed by the project.

Written by the indexing model from the issue text.