Rules to run rustdoc on multiple crates

I would like to take multiple crates and build docs for all of them in a similar fashion to [Cargo](https://doc.rust-lang.org/cargo/commands/cargo-doc.html).

With Cargo, I'm able to take the following workspace with packages, `foo`, `bar`, and `baz`, where `foo` depends on `bar` and `baz` is an isolated library and generate all docs in one place.

[rustdoc_workspace.zip](https://github.com/bazelbuild/rules_rust/files/10726671/rustdoc_workspace.zip)
```
├── Cargo.lock
├── Cargo.toml
├── bar
│   ├── Cargo.toml
│   └── src
│   └── lib.rs
├── baz
│   ├── Cargo.toml
│   └── src
│   └── lib.rs
└── foo
├── Cargo.toml
└── src
├── lib.rs
└── main.rs

6 directories, 9 files
```