Extending single entry endpoint's `links`

[merkys]

During the 2025 workshop discussions of the cheminformatics subgroup (@ijbruno @ml-evs @tomlunderwood @vaitkus @dShado94) there has been a suggestion to further extend the links of single entry endpoint. Currently single entry inherits common link types:

• base_url (OPTIMADE implementation base URL)
• describedby (schema link)
• next, prev, first, last (pagination)
• self (self-link)

There has been suggestions to add at least the following:

• webpage/landing_page/html (pointing to a human-readable landing webpage for the entry; there were no consensus as to how to call such field)
• doi (pointing to DOI of this OPTIMADE entry, must be prefixed with DOI resolver URL)

(there might have been more suggestions, feel free to add)

[merkys]

Also there have been suggestions to include a list of external identifiers (only appropriate under links for identifiers as URIs) and related entries in other OPTIMADE providers.

These were the examples I made during the discussion session:

{
  "self": "...",
  "doi": "...",
  "landing_page": "https://...",
  "identifiers": [],
  "related": {
    "cod": [ "https://...", "...", "...", "..." ],
    "csd": [ "https://...", "..." ],
    "pdb": [],
    // ...
  }
}

{
  "self": "...",
  "doi": "...",
  "landing_page": "https://...",
  "identifiers": [],
  "related": {
    "optimade": [],
    "other": []
  }
}

The latter one I think is more appropriate, as from OPTIMADE URL it should be easy to identify the providing database, while in the former a controlled vocabulary is supposed.