docs(s3-utils): add comprehensive README with API reference

[pyramation]

Summary

Adds full documentation to the @constructive-io/s3-utils README covering:

• Package overview and feature list
• Installation and quick-start example
• Complete API reference for all exported functions: createS3Client, presignPutUrl, presignGetUrl, headObject, fileExists, upload, download, uploadThrough, createS3Bucket
• StorageConnectionConfig options table with provider-specific behavior notes
• Related packages table linking sibling uploads/ packages

Review & Testing Checklist for Human

• Verify API signatures in the README match the actual source code
• Confirm the related packages table links are correct

Notes

Documentation-only change — no code modifications.

Link to Devin session: https://app.devin.ai/sessions/c4e9279cd58d4ea0ab8c4c3d32ed00d7
Requested by: @pyramation

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