More general references in datacite.yml

The `references` field in the datacite file should be more general with respect to the reference ID.
```yaml
references:
-
id: "xxxxxx"
reftype: "IsSupplementTo"
description: "Publication name or description"
```
The explanation text should explain that ID could be a DOI, URL, ISBN, arXiv, or PMID. They should be provided in the form `type:ID` e.g., `doi:10.xxx/zzzz`, `pmid:xxxyyy`, etc.

Update the validator in G-Node/gin-doi to accept this form.

Start by inspecting the references example in datacite.yml, then locate the validator in G-Node/gin-doi. Confirm how reference IDs are currently parsed and identify the existing validation checks. Done means the explanation documents type:ID values such as DOI, URL, ISBN, arXiv, and PMID, and the validator accepts that form.

Written by the indexing model from the issue text.