docs: clarify region requirement for S3-compatible endpoints

[davidmyriel]

Hey there! Small docs fix for the S3 connector.

The region field is marked required in the config table, but the S3-compatible examples (MinIO, R2, Spaces) never say what to put there. Since those services don't use AWS regions, users either guess an AWS region or get stuck wondering why the field is asking for something that doesn't apply to their setup.

This PR:

• Adds a one-liner explaining that region: 'auto' works for S3-compatible endpoints (the value is still needed for request signing, but the service ignores it)
• Updates the MinIO example to use auto instead of us-east-1
• Annotates the R2 and Spaces endpoint lines with the same hint
• Adds Tigris to the list of S3-compatible providers while I was in there — same endpoint pattern as the others

No code changes, no new pages, just making an existing doc a bit clearer. Happy to adjust wording if you'd rather keep the scope tighter.

Thanks for taking a look!