Import the `{input, output, error}` modules in the RustDocs service stub example (#2096)

Instead of importing the types within those modules. It's thus easier to
see what the types' roles are when using them in arbitrary business
logic.

Before:

```rust
use my_service::input::Foo;
use my_service::output::Bar;

async fn my_handler(input: Foo) -> Bar { todo!() }
```

With this commit:

```rust
use my_service::{input, output};

async fn my_handler(input: input::Foo) -> output::Bar { todo!() }
```