Add `NodeRelationship.PARENT` in llama_index

### Requested feature

When I save a docling document as JSON, it has nice references for parents, children, and groups. If I load this data into a vector database with llama_index, the prev/next/source relationships are there, but the parent relationship is gone. Dumping the node returned for a RAG query returns a `relationships` key:

```
relationships={
: RelatedNodeInfo(
: RelatedNodeInfo(
: RelatedNodeInfo(
```

But the parent relationship is missing and that causes `node.parent_node` to be empty.

### Alternatives

I could go digging through metadata for the node to get the parent, build a metadata filter, and have llama_index search the vector DB for that, but it feels inefficient and error prone.

Should I be going about this in another way?