Recipe: calculate age from dob in external file

https://forum.getodk.org/t/calculate-age-from-external-csv/46217/6?u=ln

Sadly this is particularly annoying because we haven't addressed type coercion bugs around dates in JR.

Start with the linked forum discussion to understand the requested recipe and the date type-coercion limitation mentioned in the issue. Then locate the relevant recipe documentation in the Sphinx-based docs repository. Done means documenting how to calculate age from a date of birth stored in an external CSV.

Written by the indexing model from the issue text.