Document the use of external Minio for object storage

Summary

We currently provide documentation for using S3 via AWS. Using an external Minio instance works in a very similar way, but requires 3 key items that are not currently documented well, and will likely trip users up.

Actionable items

• Add examples/objectstorage/rails.minio.yml based on rails.minio.yml
• Add examples/objectstorage/registry.minio.yml based on registry.minio.yml
• Add doc/advanced/external-object-storage/minio.md documenting the use of an external Minio server, and the need to ensure the buckets are created.

The key items that need to be added are to the example yamls:

• endpoint: protocol://fqdn:port
• host: fqdn
• path_style: true