feat: KG RAG query engine, enable Graph RAG on existing KGs (#7204)

CHANGELOG.md

.../query/query_engines/knowledge_graph_rag_query_engine.rst

docs/core_modules/query_modules/retriever/modules.md

.../examples/query_engine/knowledge_graph_query_engine.ipynb

...mples/query_engine/knowledge_graph_rag_query_engine.ipynb

llama_index/graph_stores/nebulagraph.py

llama_index/indices/knowledge_graph/__init__.py

llama_index/indices/knowledge_graph/base.py

llama_index/indices/knowledge_graph/retrievers.py

llama_index/query_engine/__init__.py

llama_index/query_engine/knowledge_graph_query_engine.py

llama_index/retrievers/__init__.py

tests/indices/knowledge_graph/test_retrievers.py