docs: Nimble provider doc fixes (#29597)

## Description

- Removed broken link for the API Reference
- Added `OPENAI_API_KEY` setter for the chains to properly run
- renamed one of our examples so it won't override the original
retriever and cause confusion due to it using a different mode of
retrieving
- Moved one of our simple examples to be the first example of our
retriever :)

docs/docs/integrations/providers/nimble.ipynb

@@ -19,7 +19,7 @@
     "id": "72ee0c4b-9764-423a-9dbf-95129e185210"
    },
    "source": [
-    "# Nimbleway\n",
+    "# Nimble\n",
     "\n",
     "  [Nimble](https://www.linkedin.com/company/nimbledata) is the first business external data platform, making data decision-making easier than ever, with our award-winning AI-powered data structuring technology Nimble connects business users with the public web knowledge.\n",
     "We empower businesses with mission-critical real-time external data to unlock advanced business intelligence, price comparison, and other public data for sales and marketing. We translate data into immediate business value.\n",