improve docs for indexes (#1146)

docs/modules/indexes.rst

@@ -0,0 +1,25 @@
+Indexes
+==========================
+
+Indexes refer to ways to structure documents so that LLMs can best interact with them.
+This module contains utility functions for working with documents, different types of indexes, and then examples for using those indexes in chains.
+LangChain provides common indices for working with data (most prominently support for vector databases).
+For more complicated index structures, it is worth checking out `GPTIndex <https://gpt-index.readthedocs.io/en/latest/index.html>`_.
+
+The following sections of documentation are provided:
+
+- `Getting Started <./indexes/getting_started.html>`_: An overview of all the functionality LangChain provides for working with indexes.
+
+- `Key Concepts <./indexes/key_concepts.html>`_: A conceptual guide going over the various concepts related to indexes and the tools needed to create them.
+
+- `How-To Guides <./indexes/how_to_guides.html>`_: A collection of how-to guides. These highlight how to use all the relevant tools, the different types of vector databases, and how to use indexes in chains.
+
+
+.. toctree::
+   :maxdepth: 1
+   :name: LLMs
+   :hidden:
+
+   ./indexes/getting_started.ipynb
+   ./indexes/key_concepts.md
+   ./indexes/how_to_guides.rst