Document json files like InterfaceData.json and GroupData.json

[teoli2003]

@peterbe is migrating many JSON files from mdn/yari to mdn/content in mdn/yari#4081

These files, created as an experiment, were never documented. As the process to update them is slightly changing, it is a good time to document their structure and their usages, and to explain how to maintain them.

I'm filling this issue not to forgot to do this. (I co-created them half a decade ago.)

Importation in mdn/content is done by #8019