Kamal 2.8.0 introduced a local docker registry to push and pull app images, see: #1355. This PR updates the registry configuration documentation to reflect these changes. It also fixes some minor inconsistencies in the configuration examples.

I believe the changes help Kamal users with the migration to a local docker registry.

Key changes:

• Moved Docker Hub to its own section.
• Replaced DOCKER_REGISTRY_TOKEN with KAMAL_REGISTRY_PASSWORD to be consistent with the .kamal/secrets file.
• Replaced the username example to be consistent with the other examples.
• Removed server: registry.digitalocean.com from that section. I found that confusing in a Docker Hub section.
• Added a local container registry section with a configuration example. Mostly for users migrating to a local registry.