Use the correct documentation

Seems the correct documentation is now here:
- https://inveniordm.docs.cern.ch/reference/metadata/

And not in:
- https://developers.zenodo.org/

Should use the schema from:
- https://schema.datacite.org/meta/kernel-4.6/

Start by searching the repository for references to developers.zenodo.org and the metadata schema, then compare them with the InvenioRDM metadata documentation and DataCite Kernel 4.6 links in the issue. Done means the relevant documentation or schema references use the sources identified here and no outdated reference remains.

Written by the indexing model from the issue text.