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/ecosystem/serpapi.md

@@ -0,0 +1,31 @@
+# SerpAPI
+
+This page covers how to use the SerpAPI search APIs within LangChain.
+It is broken into two parts: installation and setup, and then references to specific Pinecone wrappers.
+
+## Installation and Setup
+- Install requirements with `pip install google-search-results`
+- Get a SerpAPI api key and either set it as an environment variable (`SERPAPI_API_KEY`)
+
+## Wrappers
+
+### Utility
+
+There exists a SerpAPI utility which wraps this API. To import this utility:
+
+```python
+from langchain.utilities import SerpAPIWrapper
+```
+
+For a more detailed walkthrough of this wrapper, see [this notebook](../modules/utils/examples/serpapi.ipynb).
+
+### Tool
+
+You can also easily load this wrapper as a Tool (to use with an Agent).
+You can do this with:
+```python
+from langchain.agents import load_tools
+tools = load_tools(["serpapi"])
+```
+
+For more information on this, see [this page](../modules/agents/tools.md)