docs: compound ecosystem and integrations (#4870)

# Docs: compound ecosystem and integrations

**Problem statement:** We have a big overlap between the
References/Integrations and Ecosystem/LongChain Ecosystem pages. It
confuses users. It creates a situation when new integration is added
only on one of these pages, which creates even more confusion.
- removed References/Integrations page (but move all its information
into the individual integration pages - in the next PR).
- renamed Ecosystem/LongChain Ecosystem into Integrations/Integrations.
I like the Ecosystem term. It is more generic and semantically richer
than the Integration term. But it mentally overloads users. The
`integration` term is more concrete.
UPDATE: after discussion, the Ecosystem is the term.
Ecosystem/Integrations is the page (in place of Ecosystem/LongChain
Ecosystem).

As a result, a user gets a single place to start with the individual
integration.

docs/integrations/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 the specific SerpAPI wrapper.
+
+## 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/agents/tools/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/getting_started.md)