Add links to CMOR definitions

[znichollscr]

The CMOR definitions are very opaque to people who don't work with this stuff a lot. For example, what the grid labels actually mean, which is actually spelt out pretty nicely here: https://github.com/PCMDI/mip-cmor-tables/blob/main/MIP_grid_label.json

To close this issue:

• [ ] add a section to the README which collates these key links
• [ ] point to that section in the README anywhere we think is sensible

The long-term solution would be to link docs on in the CMOR/MIP repositories where this is captured, but I don't think they're stable/available yet (@durack1 please correct me if I'm wrong).

x-ref https://github.com/PCMDI/input4MIPs_CVs/issues/55#issuecomment-2291333561

[znichollscr]

1. Look at Table 1 of https://docs.google.com/document/d/1h0r8RZr_f3-8egBMMh7aqLwy3snpD6_MrDz1q8n5XUk/edit. The descriptions and examples columns are most helpful
2. Worth then also checking https://github.com/PCMDI/mip-cmor-tables, to see if there is updated information for the attribute of interest
3. Hopefully a lot of the extra metadata will then flow through to https://github.com/WCRP-CMIP/CMIP6Plus_CVs

Over time, mip CMOR tables should become the source of truth. For now, it's not quite so crystal clear.

There is heaps of information available. The issue is that it's not in easily usable, codified form (e.g. as a Fortran/python library). History books with all the information: https://zenodo.org/communities/pcmdi/records?q=&l=list&p=1&s=10&sort=newest

If we can get the JSON-LD stuff going (x-ref #110), then we can likely auto-generate a lot of this information.