docs: add Tigris to S3-compatible providers

[davidmyriel]

Summary

Hey! 👋 This PR adds Tigris as an example S3-compatible provider to the existing Amazon S3 file storage docs.

• Adds "Tigris" to the list of S3-compatible services in the AWS_ENDPOINT_URL description
• Adds an "S3-Compatible Providers" section at the bottom of the S3 page with a Tigris .env example — just the values that differ from the standard AWS setup
• The section is structured so other providers' examples can be added in the same format

Why

MinIO's open-source repo was archived in February 2026, and users looking for S3-compatible alternatives need documented options. The page already mentions MinIO, R2, Hetzner, and Backblaze B2 by name — this adds Tigris with a concrete configuration example so users can get started quickly.

Test plan

• Verified env var names match the codebase (AWS_ENDPOINT_URL, AWS_REGION, AWS_FORCE_PATH_STYLE in packages/api/src/storage/s3/s3Config.ts and packages/api/src/cdn/s3.ts)
• No new files — single-page edit to content/docs/configuration/cdn/s3.mdx
• Docs build cleanly

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