feat(router): setup router query engine (#484)

apps/docs/docs/modules/query_engines/_category_.yml

apps/docs/docs/modules/query_engines/index.md

apps/docs/docs/modules/query_engines/router_query_engine.md

examples/routerQueryEngine.ts

packages/core/src/OutputParser.ts

packages/core/src/Prompt.ts

packages/core/src/QuestionGenerator.ts

packages/core/src/Response.ts

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

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

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

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

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

packages/core/src/index.ts

packages/core/src/indices/keyword/KeywordTableIndex.ts

packages/core/src/indices/summary/SummaryIndex.ts

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

packages/core/src/llm/LLM.ts

packages/core/src/outputParsers/selectors.ts

packages/core/src/selectors/base.ts