Document Cargo (Rust) packages in registry package types

The registry has supported `registryType: "cargo"` since https://github.com/modelcontextprotocol/registry/pull/1207,
and it is live in production, but this page never listed it.

The section is reflected verbatim from the source of this page:
https://github.com/modelcontextprotocol/registry/blob/main/docs/modelcontextprotocol-io/package-types.mdx