write_zenodo_json(): link the package website in the Zenodo description and related identifiers #154

The Zenodo record of a data package should point readers to the pkgdown site. On guluswm 1.0.0 this was added by hand in the Zenodo editor (description line "Explore the data on a curated website: https://openwashdata.github.io/guluswm" and a related work "Is documented by" with the site URL). The next release overwrites both, because Zenodo reads .zenodo.json from the tag.

Proposal, in .zenodo.json only (DESCRIPTION, CITATION.cff, inst/CITATION and the README stay unchanged):

• description: the Description field as the first paragraph, then a second paragraph "Explore the data on a curated website: ", with the url from _pkgdown.yml (url:), falling back to the github.io entry of the DESCRIPTION URL field. Zenodo renders the description as HTML, so write <p>...</p><p>Explore the data on a curated website: <a href="URL">URL</a></p>.
• related_identifiers: add {identifier: <url>, relation: "isDocumentedBy", resource_type: "other"}. Today the repository link is only written when the package has source DOIs; write it always, so the record keeps the GitHub link that Zenodo otherwise adds by default.
• Skip both when no site URL is found.

Tests: a package with and without _pkgdown.yml, and one with source DOIs, so the repository link and isDerivedFrom stay as they are.