docs: clarify Bright Data API key setup instructions (#1201)

## Overview
Added clearer instructions on how to get API key's from Bright Data.

## Type of change
**Type:** Update existing documentation

## Related issues/PRs
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## Checklist
<!-- Put an 'x' in all boxes that apply -->
- [X] I have read the [[contributing guidelines](README.md)
- [X] I have tested my changes locally using `docs dev`
- [X] All code examples have been tested and work correctly
- [ ] I have used **root relative** paths for internal links
- [ ] I have updated navigation in `src/docs.json` if needed
- [ ] I have gotten approval from the relevant reviewers
- [ ] (Internal team members only / optional) I have created a preview
deployment using the [[Create Preview Branch
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## Additional notes
This change improves the setup instructions by providing a concrete
Python code example for setting the API key environment variable, making
it easier for users to get started with langchain-brightdata.

Author: meirk-brd

src/oss/python/integrations/providers/brightdata.mdx

@@ -20,6 +20,8 @@ uv add langchain-brightdata
 
 You'll need to set up your Bright Data API key:
 
+Sign up at [Bright Data](https://brightdata.com/?utm_source=tech-partner&utm_medium=link&utm_campaign=langchain&hs_signup=1) and retrieve your API key from your account settings. Replace `"your-api-key"` with your actual API key in the examples below: 
+
 ```python
 import os
 os.environ["BRIGHT_DATA_API_KEY"] = "your-api-key"