Add example of using a LangChain tool to the docs

[discdiver]

Enhancement Description

Would show a user how to add common pre-built tools. It took me a bit to get this to work.

Use Case

Say a user wants to get up-to-date information into their app. You can do this with the DuckDuckGoSearchRun tool.

Proposed Implementation

Create an example that shows how to use the `DuckDuckGoSearchRun` LangChain tool.

[jlowin]

I think this is done!