Response Builder/Synthesizer UX V2 (#6668)

CONTRIBUTING.md

docs/examples/agent/openai_agent_query_plan.ipynb

docs/examples/index_structs/doc_summary/DocSummary.ipynb

...phIndex_vs_VectorStoreIndex_vs_CustomIndex_combined.ipynb

docs/examples/node_postprocessor/OptimizerDemo.ipynb

docs/examples/query_engine/CustomRetrievers.ipynb

docs/examples/query_engine/citation_query_engine.ipynb

docs/guides/primer/usage_pattern.md

docs/how_to/customization/streaming.md

docs/how_to/index/vector_store_guide.ipynb

docs/how_to/query_engine/advanced/optimizers.md

docs/how_to/query_engine/advanced/root.md

docs/how_to/query_engine/advanced/second_stage.md

docs/how_to/query_engine/usage_pattern.md

docs/index.rst

docs/reference/optimizers.rst

llama_index/__init__.py

llama_index/indices/common/struct_store/base.py

llama_index/indices/document_summary/base.py

llama_index/indices/postprocessor/__init__.py