feat: Knowledge Graph Query Engine (#6642)

CHANGELOG.md

docs/api_reference/query/query_engines.rst

...ence/query/query_engines/knowledge_graph_query_engine.rst

docs/community/integrations.md

docs/community/integrations/graph_stores.md

docs/core_modules/query_modules/query_engine/modules.md

...phIndex_vs_VectorStoreIndex_vs_CustomIndex_combined.ipynb

...ndex_structs/knowledge_graph/NebulaGraphKGIndexDemo.ipynb

...mples/index_structs/knowledge_graph/nebulagraph_draw.html

.../examples/query_engine/knowledge_graph_query_engine.ipynb

docs/examples/query_engine/nebulagraph_draw.html

docs/examples/query_engine/nebulagraph_draw_quill.html

llama_index/graph_stores/nebulagraph.py

llama_index/graph_stores/simple.py

llama_index/graph_stores/types.py

llama_index/prompts/prompt_type.py

llama_index/query_engine/__init__.py

llama_index/query_engine/knowledge_graph_query_engine.py