Build API docs

[hydroper]

Did you think of something like cargo doc? That will build API docs locally.

Also, crates.io automatically builds docs in a Docker instance and shares its link at the package repository. But I think this is complex, so supporting poac doc is enough.

[ken-matsui]

@hydroper Yes, that feature is definitely on the roadmap, although its priority is low. Most features of Cargo will be covered eventually.