Add simple hello.md file

[devin-ai-integration]

Summary

Adds a simple hello.md file to the repository root as a first step, as requested by the user.

Review & Testing Checklist for Human

• Confirm the file location (root directory) is the intended placement, or if it should be moved to the fern/ documentation structure

Notes

• This is a minimal placeholder file and is not integrated into the documentation navigation
• Link to Devin run: https://app.devin.ai/sessions/d2622e780c7e4abc8460120690d45589
• Requested by: unknown ()

[devin-ai-integration]

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