Add JSON-LD representations of CVs

[znichollscr]

The JSON-LD branch will soon be merged into https://github.com/PCMDI/mip-cmor-tables

We should use this as an example of how to capture our CVs as JSON-LD, and how to link our CVs into the 'global' tables.

[durack1]

This and #203 duplicate one another.

Move CVs to esgvoc style

Example to follow here: https://github.com/WCRP-CMIP/CMIP6Plus_CVs/tree/esgvoc