app/dcs/api/s3/s3-compatibility: update with the latest storj.io/gate…

…way/docs/s3-compatibility.md

This updates the page with the latest
storj.io/gateway/docs/s3-compatibility.md to make it the ground truth
and prepare for removal of storj.io/gateway/docs/s3-compatibility.md. It
also adds another context for GetBucketLocation from
#285, superseding this pull request.