Improve docs on CSV fields

[slint]

We can integrate the following bullets into the main docs of the CSV fields:

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• Each line represents a record that will be created on Zenodo
• Required fields are marked as bold in the header. Fields that don’t have a value are skipped.
• For the doi field:
  • It should be filled in if there is a DOI already registered for an entry
  • If not filled, we’ll register a Zenodo DOI for the record
• You’ll notice that the fields are a somewhat “de-normalized” version of the JSON representation we’re using on Zenodo. Since we’re often dealing with “complex” fields such as multi-level nesting of arrays of objects, we have taken some liberty with the data formatting to allow representing these values. Some examples of such fields:
  • Keywords (subjects.subject): the cell value contains “new-line” separated keywords
  • Creators/authors (creators.*): following the “new-line” separated convention, these have been “tabularized”. In the example there are two authors: Nils Schlüter (affiliation: Museum für Naturkunde, ORCID: 0000-0002-5699-3684) and John Smith (affiliation: CERN, ORCID: none)
• Some of the fields rely on controlled vocabularies (e.g. the resource types, contributor types, licenses, related identifier relation types, etc.). The values for these types can be found under the following endpoints (to which you can add a ?q=<search term> query string parameter to narrow down results)
  • Resource Type ID ([resource_type.id](http://resource_type.id/)): https://zenodo.org/api/vocabularies/resourcetypes?size=1000
  • Creator affiliations ID ([creators.affiliations.id](http://creators.affiliations.id/)): https://zenodo.org/api/affiliations
    • Basically we accept a valid ROR ID
  • Rights/License ID ([rights.id](http://rights.id/)): https://zenodo.org/api/vocabularies/licenses?size=1000
    • These are all based on SPDX IDs, but we have some “custom” cases for e.g. “Other (open)”, etc. that are available at the endpoint
  • Contributor Role ID ([contributors.role.id](http://contributors.role.id/)): https://zenodo.org/api/vocabularies/contributorsroles?size=1000
  • Language IDs ([languages.id](http://languages.id/)): https://zenodo.org/api/vocabularies/languages?size=1000
  • Related identifiers Relation Type ID ([related_identifiers.relation_type.id](http://related_identifiers.relation_type.id/)): https://zenodo.org/api/vocabularies/relationtypes?size=1000
• For custom fields we have a reference sheet at https://docs.google.com/spreadsheets/d/1TUyDT6yOypX2DBuM_PNUZucFTC93uFlEa7PoAMYvnDI/edit#gid=314238332, but the basic premise is that they correspond to known vocabularies such as DarwinCore, AudubonCore, etc. They all receive multiple terms