In JSON:API docs the "Fetching Relationships" section says:

The top-level links object MAY contain self and related links, as described above for relationship objects.

And it gives the example response:

{
  "links": {
    "self": "/articles/1/relationships/author",
    "related": "/articles/1/author"
  },
  "data": {
    "type": "people",
    "id": "12"
  }
}

However, it seems like this library does not allow to generate such document. Here's my attempt:

new JsonApi\DataDocument(
    new JsonApi\ResourceIdentifier("people", "12"),
    new JsonApi\Link\SelfLink("/articles/1/relationships/author")
);

It gives:

{
    "data": {
        "type": "people",
        "id": "12"
    },
    "links": {
        "self": "/articles/1/relationships/author"
    }
}

But the related link can't be added because DataDocument constructor does not accept RelatedLink objects.