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/utils.rst

@@ -0,0 +1,26 @@
+Utilities
+==============
+
+There are a lot of different utilities that LangChain provides integrations for
+These guides go over how to use them.
+These can largely be grouped into two categories: generic utilities, and then utilities for working with larger text documents.
+
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+   :caption: Generic Utilities
+
+   modules/python
+   modules/serpapi
+
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+   :caption: Utilities for working with Documents
+
+   modules/docstore
+   modules/text_splitter
+   modules/embeddings
+   modules/vectorstore