beef up retrieval docs (#9518)

docs/docs_skeleton/docs/modules/data_connection/index.mdx

@@ -2,15 +2,60 @@
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-# Data connection
+# Retrieval
 
-Many LLM applications require user-specific data that is not part of the model's training set. LangChain gives you the 
-building blocks to load, transform, store and query your data via:
+Many LLM applications require user-specific data that is not part of the model's training set.
+The primary way of accomplishing this is through Retrieval Augmented Generation (RAG).
+In this process, external data is *retrieved* and then passed to the LLM when doing the *generation* step.
 
-- [Document loaders](/docs/modules/data_connection/document_loaders/): Load documents from many different sources
-- [Document transformers](/docs/modules/data_connection/document_transformers/): Split documents, convert documents into Q&A format, drop redundant documents, and more
-- [Text embedding models](/docs/modules/data_connection/text_embedding/): Take unstructured text and turn it into a list of floating point numbers
-- [Vector stores](/docs/modules/data_connection/vectorstores/): Store and search over embedded data
-- [Retrievers](/docs/modules/data_connection/retrievers/): Query your data
+LangChain provides all the building blocks for RAG applications - from simple to complex.
+This section of the documentation covers everything related to the *retrieval* step - e.g. the fetching of the data.
+Although this sounds simple, it can be subtly complex.
+This encompasses several key modules.
 
 ![data_connection_diagram](/img/data_connection.jpg)
+
+**[Document loaders](/docs/modules/data_connection/document_loaders/)**
+
+Load documents from many different sources.
+LangChain provides over a 100 different document loaders as well as integrations with other major providers in the space,
+like AirByte and Unstructured.
+We provide integrations to load all types of documents (html, PDF, code) from all types of locations (private s3 buckets, public websites).
+
+**[Document transformers](/docs/modules/data_connection/document_transformers/)**
+
+A key part of retrieval is fetching only the relevant parts of documents.
+This involves several transformation steps in order to best prepare the documents for retrieval.
+One of the primary ones here is splitting (or chunking) a large document into smaller chunks.
+LangChain provides several different algorithms for doing this, as well as logic optimized for specific document types (code, markdown, etc).
+
+**[Text embedding models](/docs/modules/data_connection/text_embedding/)**
+
+Another key part of retrieval has become creating embeddings for documents.
+Embeddings capture the semantic meaning of text, allowing you to quickly and
+efficiently find other pieces of text that are similar.
+LangChain provides integrations with over 25 different embedding providers and methods,
+from open-source to proprietary API,
+allowing you to choose the one best suited for your needs.
+LangChain exposes a standard interface, allowing you to easily swap between models.
+
+**[Vector stores](/docs/modules/data_connection/vectorstores/)**
+
+With the rise of embeddings, there has emerged a need for databases to support efficient storage and searching of these embeddings.
+LangChain provides integrations with over 50 different vectorstores, from open-source local ones to cloud-hosted proprietary ones,
+allowing you choose the one best suited for your needs.
+LangChain exposes a standard interface, allowing you to easily swap between vector stores.
+
+**[Retrievers](/docs/modules/data_connection/retrievers/)**
+
+Once the data is in the database, you still need to retrieve it.
+LangChain supports many different retrieval algorithms and is one of the places where we add the most value.
+We support basic methods that are easy to get started - namely simple semantic search.
+However, we have also added a collection of algorithms on top of this to increase performance.
+These include:
+
+- [Parent Document Retriever](/docs/modules/data_connection/retrievers/parent_document_retriever): This allows you to create multiple embeddings per parent document, allowing you to look up smaller chunks but return larger context.
+- [Self Query Retriever](/docs/modules/data_connection/retrievers/self_query): User questions often contain reference to something that isn't just semantic, but rather expresses some logic that can best be represented as a metadata filter. Self-query allows you to parse out the *semantic* part of a query from other *metadata filters* present in the query
+- [Ensemble Retriever](/docs/modules/data_connection/retrievers/ensemble): Sometimes you may want to retrieve documents from multiple different sources, or using multiple different algorithms. The ensemble retriever allows you to easily do this.
+- And more!
+