[DOC] Index of quality metrics

[oesteban]

Write an index where all IQMs point to their name and definition in a more clear way

[riddlet]

We created a data dictionary for NDA where we attempted to provide definitions for everything:

https://nda.nih.gov/data_structure.html?short_name=mriqm01

Note that there are a handful of IQM's whose name we had to modify to keep them under 30 characters, but we hope this can be of use.

EDIT: Relatedly, our group needs a function that will produce files consistent with this nda data dictionary. Is this something that mriqc would be interested in integrating? I can issue a pull request that will create this functionality.