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/generic_how_to.rst

@@ -0,0 +1,20 @@
+Generic Functionality
+=====================
+
+The examples here all address certain "how-to" guides for working with LLMs.
+
+`LLM Serialization <examples/llm_serialization.html>`_: A walkthrough of how to serialize LLMs to and from disk.
+
+`LLM Caching <examples/llm_caching.html>`_: Covers different types of caches, and how to use a cache to save results of LLM calls.
+
+`Custom LLM <examples/custom_llm.html>`_: How to create and use a custom LLM class, in case you have an LLM not from one of the standard providers (including one that you host yourself).
+
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+   :caption: Generic Functionality
+   :name: Generic Functionality
+   :hidden:
+
+   examples/*