feat: multi-document agents (#531)

.changeset/thin-seals-cough.md

apps/docs/docs/modules/agent/_category_.yml

apps/docs/docs/modules/agent/multi_document_agent.mdx

apps/docs/docs/modules/agent/openai.mdx

apps/docs/docs/modules/agent/query_engine_tool.mdx

examples/.gitignore

examples/agent/helpers/extractWikipedia.ts

examples/agent/multi_document_agent.ts

examples/agent/query_openai_agent.ts

examples/vectorIndexEmbed3.ts

packages/core/package.json

packages/core/src/agent/openai/base.ts

packages/core/src/agent/openai/worker.ts

packages/core/src/agent/react/base.ts

packages/core/src/agent/react/worker.ts

packages/core/src/engines/query/SubQuestionQueryEngine.ts

packages/core/src/index.ts

packages/core/src/indices/vectorStore/VectorStoreIndex.ts

packages/core/src/llm/LLM.ts

packages/core/src/llm/openai/utils.ts