Integrate the essential model documentation with the catalog

[sturoscy-personal]

In the old system, it was a further_info field in the catalog schema that contained a link to the essential model doc web application

[sashakames]

@davidhassell or @wolfiex may know. If EMD is accessible via source_id then the publisher could generate links provided we can configure a template to use.

[glevava]

The EMD is a significant complementary source of information associated with each CMIP7 source_id, and is stored in the corresponding JSON file in the CMIP7 CVs GitHub repo. I agree with @sashakames as such, a link can be easily derived by Metagrid to access the relevant JSON document. An example can be found here (although without the EMD content itself):
https://github.com/WCRP-CMIP/CMIP7-CVs/blob/esgvoc/source/canesm6-mr.json

That said, I am not fully convinced of the added value of directly pointing users to the raw JSON file itself. It is very verbose and not particularly user-friendly.

For CMIP6, the further_info_url pointed to the ES-DOC model comparison service, which allowed users to compare models property by property. That comparative view was really the key value of the service. As CMIP7 would not have an equivalent service available, the attribute has been dropped. @davidhassell may be able to provide more insight on that.

As an interim compromise, while waiting for a potential successor to ES-DOC, we will soon be putting into production at IPSL a CV explorer: https://esgvoc.ipsl.fr/concept/source. This front-end could be used to present source_id information in a more accessible way, even if it does not yet support direct comparison between models. It provides a more user-friendly view of the underlying JSON and could also help users become more familiar with the broader CMIP CV concepts, and the links are fully predictable for Metagrid.

[glevava]

See my comment in #144