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/utils.rst

@@ -0,0 +1,26 @@
+Utils
+==========================
+
+While LLMs are powerful on their own, they are more powerful when connected with other sources of knowledge or computation.
+This section highlights those sources of knowledge or computation,
+and goes over how to easily use them from within LangChain.
+
+The following sections of documentation are provided:
+
+- `Key Concepts <utils/key_concepts.html>`_: A conceptual guide going over the various types of utils.
+
+- `How-To Guides <utils/how_to_guides.html>`_: A collection of how-to guides. These highlight how to use various types of utils.
+
+- `Reference </reference/utils.html>`_: API reference documentation for all Util classes.
+
+
+
+.. toctree::
+   :maxdepth: 1
+   :caption: Utils
+   :name: Utils
+   :hidden:
+
+   utils/key_concepts.md
+   utils/how_to_guides.rst
+   Reference </reference/utils.rst>