Show concrete example of querying a CSV dataset

At https://docs.getodk.org/form-datasets/#referencing-values-in-datasets

Feedback from https://forum.getodk.org/t/last-saved-value-does-not-work-as-dynamic-default-with-csv-search-how-to-replace-csv-search-by-select-one-from-file/32046/5?u=ln

Start at the “Referencing values in datasets” section linked in the issue and review the linked forum discussion for the requested context. Add a concrete example of querying a CSV dataset, then verify the rendered documentation clearly explains the example and its expected usage.

Written by the indexing model from the issue text.