ref/publish: self-hosting

[casperdcl]

Following iterative/cml#1075 <- iterative/cml#291,

• Document cml publish --url #317
  • cml publish --url="https://images.mydomain.com/" ./plot.png
  • or CML_PUBLISH_URL="https://images.mydomain.com/" cml publish ./plot.png
• Release self-host server code
  • https://github.com/iterative/minroud-s3
  • https://github.com/0x2b3bfa0/minroud-r2 (explicitly excluded)
• Merge Introduce subcommands in a backwards-compatible way cml#1073 (not relevant)