Document about the schema

[vchrombie]

ELK keeps a description for each enriched data used to build the KIbiter dashboards. Such descriptions are stored in the folder schema as CSV files.

Most of the schema of the data sources are not updated, or not described sufficiently. There is no documentation around this too. I think we need to have some proper documentation about this and think about how to keep these updated regularly. This is the aim of this issue.