docs: add S3-compatible alternatives section with Tigris example

[davidmyriel]

Summary

Hey! 👋 This PR adds a small subsection to the existing S3 storage tutorial showing how to use S3-compatible providers other than AWS.

• Adds an "S3-Compatible Alternatives" section at the end of s3-storage.md explaining that the same s3 storage provider works with any S3-compatible service
• Includes Tigris as the first example — just the two env vars that differ (S3_ENDPOINT_URL and S3_REGION_NAME), no duplicate docker commands
• Adds "Tigris" to the S3-compatible provider list in scaling.md alongside MinIO and R2

The section is structured so other providers (Backblaze B2, Ceph, etc.) can be added below in the same format.

Why

MinIO's open-source repo was archived in February 2026, and a few users have been asking about alternatives. Tigris is S3-compatible with zero egress fees and a free tier, so it's a natural fit. Rather than a separate tutorial page, this keeps everything in one place and just highlights what's different from the standard AWS setup.

[pr-validator-bot]

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