Docs refactor (#480)

Big docs refactor! Motivation is to make it easier for people to find
resources they are looking for. To accomplish this, there are now three
main sections:

- Getting Started: steps for getting started, walking through most core
functionality
- Modules: these are different modules of functionality that langchain
provides. Each part here has a "getting started", "how to", "key
concepts" and "reference" section (except in a few select cases where it
didnt easily fit).
- Use Cases: this is to separate use cases (like summarization, question
answering, evaluation, etc) from the modules, and provide a different
entry point to the code base.

There is also a full reference section, as well as extra resources
(glossary, gallery, etc)

Co-authored-by: Shreya Rajpal <ShreyaR@users.noreply.github.com>

docs/modules/chains.rst

@@ -0,0 +1,29 @@
+Chains
+==========================
+
+Using an LLM in isolation is fine for some simple applications,
+but many more complex ones require chaining LLMs - either with eachother or with other experts.
+LangChain provides a standard interface for Chains, as well as some common implementations of chains for easy use.
+
+The following sections of documentation are provided:
+
+- `Getting Started <chains/getting_started.html>`_: A getting started guide for chains, to get you up and running quickly.
+
+- `Key Concepts <chains/key_concepts.html>`_: A conceptual guide going over the various concepts related to chains.
+
+- `How-To Guides <chains/how_to_guides.html>`_: A collection of how-to guides. These highlight how to use various types of chains.
+
+- `Reference </reference/chains.html>`_: API reference documentation for all Chain classes.
+
+
+
+.. toctree::
+   :maxdepth: 1
+   :caption: Chains
+   :name: Chains
+   :hidden:
+
+   chains/getting_started.ipynb
+   chains/how_to_guides.rst
+   chains/key_concepts.rst
+   Reference</reference/modules/chains.rst>