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/llms.rst

@@ -0,0 +1,27 @@
+LLMs
+==========================
+
+Large Language Models (LLMs) are a core component of LangChain.
+LangChain is a provider of LLMs, but rather provides a standard interface through which
+you can interact with a variety of LLMs.
+
+The following sections of documentation are provided:
+
+- `Getting Started <llms/getting_started.html>`_: An overview of all the functionality the LangChain LLM class provides.
+
+- `Key Concepts <llms/key_concepts.html>`_: A conceptual guide going over the various concepts related to LLMs.
+
+- `How-To Guides <llms/how_to_guides.html>`_: A collection of how-to guides. These highlight how to accomplish various objectives with our LLM class, as well as how to integrate with various LLM providers.
+
+- `Reference </reference/modules/llms.html>`_: API reference documentation for all LLM classes.
+
+
+.. toctree::
+   :maxdepth: 1
+   :name: LLMs
+   :hidden:
+
+   llms/key_concepts.md
+   llms/getting_started.ipynb
+   llms/how_to_guides.rst
+   Reference</reference/modules/llms.rst>