docs: contributed Graph RAG Retriever integration (#29744)

**Description:** 

This adds the `Graph RAG` Retriever integration documentation, per
https://python.langchain.com/docs/contributing/how_to/integrations/.

* The integration exists in this public repository:
https://github.com/datastax/graph-rag
* We've implemented the standard langchain tests for retrievers:
https://github.com/datastax/graph-rag/blob/main/packages/langchain-graph-retriever/tests/test_langchain.py
* Our integration is published to PyPi:
https://pypi.org/project/langchain-graph-retriever/

libs/packages.yml

@@ -394,3 +394,9 @@ packages:
   repo: lunary-ai/langchain-abso
   path: .
   downloads: 0
+- name: langchain-graph-retriever
+  name_title: 'Graph RAG'
+  repo: datastax/graph-rag
+  path: packages/langchain-graph-retriever
+  downloads: 0
+  provider_page: graph_rag