docs: clarify Bright Data API key setup instructions

[meirk-brd]

Overview

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

Type of change

Type: Update existing documentation

Related issues/PRs

• GitHub issue: N/A
• Feature PR: N/A

• Linear issue: N/A
• Slack thread: N/A

Checklist

• I have read the [contributing guidelines
• I have tested my changes locally using docs dev
• 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 workflow](https://github.com/langchain-ai/docs/actions/workflows/create-preview-branch.yml)

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.