docs: Improve Bright Data setup instructions

[meirk-brd]

Summary

• Improved setup instructions to clearly guide users to retrieve API key from account settings and set the BRIGHT_DATA_API_KEY environment variable

Changes

• Updated brightdata-tools.mdx Environment Variables section with clearer signup and configuration instructions

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[!NOTE] Clarifies Bright Data signup and API key environment variable setup in docs/en/tools/web-scraping/brightdata-tools.mdx.

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