Is there any way to use it directly in the source w/o having to create a Crate.toml and src dirs? Is there a getting started document? Dub recently included such a feature.

That's the hard way. `cargo new` generates both for you, so you don't need to do any of that stuff manually.

If you really, really want to, you could use https://crates.io/crates/cargo-script to do the way you're talking about.

https://doc.rust-lang.org/cargo/ has installation, getting started, a reference, all of that. It needs some work, but for the basics of doing this, it's got it all.