docs: add Diffbot readme file

docs/diffbot.md

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+<h1 align="center">Diffbot Agentic Service</h1>
+
+## Intro
+
+[Diffbot][diffbot] is a service that can extract structured data from web pages. It allows one to query a trillion pieces of connected content across the web or extract them on demand.
+
+## Pre-requisites
+
+Ensure the following environment variable is set:
+
+- `DIFFBOT_API_KEY` - API key for Diffbot service
+
+Otherwise, you can pass it in as an argument to the `DiffbotClient` constructor.
+
+### How to Retrieve API Key
+
+TODO: Add instructions
+
+[diffbot]: https://www.diffbot.com