Display parameter hierarchy description

[MattiSG]

Parameters have support for index.yaml descriptions in intermediate folders:

https://github.com/openfisca/openfisca-france/blob/ef9f177d4423d7bad755ff522b37cf894a82afd4/openfisca_france/parameters/bourses_education/bourse_lycee/index.yaml#L1-L2

https://fr.openfisca.org/legislation/bourses_education.bourse_lycee.avant_2016.increments_par_point_de_charge.10_parts?q=bourses_edu

However, this information is not displayed in the legislation explorer UI:

This information should be displayed, both for its current usefulness and for encouraging contributor to add intermediate descriptions.