Improve introduction to the tracing feature
Improve introduction to the tracing feature

Co-Authored-By: Mauko Quiroga <639565179054c86ac1aff71af1da9e675b5fb7ee@gmail.com>

openfisca/openfisca-doc,openfisca/openfisca-doc,openfisca/openfisca-doc,openfisca/openfisca-doc