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/reference.rst

@@ -0,0 +1,14 @@
+API References
+==========================
+
+All of LangChain's reference documentation, in one place.
+Full documentation on all methods, classes, and APIs in LangChain.
+
+.. toctree::
+   :maxdepth: 1
+
+   reference/prompts.rst
+   LLMs<reference/modules/llms>
+   reference/utils.rst
+   Chains<reference/modules/chains>
+   Agents<reference/modules/agents>