Revert "re-org top level"

apps/docs/docs/_static/concepts/indexing.jpg

apps/docs/docs/_static/concepts/querying.jpg

apps/docs/docs/_static/concepts/rag.jpg

apps/docs/docs/advanced.md

apps/docs/docs/api/classes/BaseEmbedding.md

apps/docs/docs/api/classes/BaseIndex.md

apps/docs/docs/api/classes/BaseNode.md

apps/docs/docs/api/classes/CallbackManager.md

apps/docs/docs/api/classes/ChatGPTLLMPredictor.md

apps/docs/docs/api/classes/CompactAndRefine.md

apps/docs/docs/api/classes/CondenseQuestionChatEngine.md

apps/docs/docs/api/classes/ContextChatEngine.md

apps/docs/docs/api/classes/Document.md

apps/docs/docs/api/classes/InMemoryFileSystem.md

apps/docs/docs/api/classes/IndexDict.md

apps/docs/docs/api/classes/IndexList.md

apps/docs/docs/api/classes/IndexNode.md

apps/docs/docs/api/classes/IndexStruct.md

apps/docs/docs/api/classes/LLMQuestionGenerator.md

apps/docs/docs/api/classes/ListIndex.md