Polish license information in documentation

Checklist

• I am using the current main branch or the latest release. Please indicate.
• I am running on an up-to-date pypsa-earth environment. Update via conda env update -f envs/environment.yaml.

Describe the Bug

For licenses, we have a summary here in the documentation but it may need some cross-checking and polishes. In particular:

• A link to Copernicus land coverage dataset is outdated and must be replaced with a newer one https://cds.climate.copernicus.eu/cdsapp/#!/terms/licence-to-use-copernicus-products -> https://zenodo.org/records/3939050
• There is a mistake for GADM whose license is by no means CC4.0, and conditions of use contain a direct restriction of re-distribution >The data are freely available for academic use and other non-commercial use. Redistribution, or commercial use is not allowed without prior permission.
• eez and copernicus landcover datasets don't have restrictions on commercial usage (since they are licensed by a standart CC4.0)