`cargo rustdoc` only works for a single package. To render docs for
a workspace while passing config options to `rustdoc`, we need to use
the `RUSTDOCFLAGS` environment variable.
We also add several other flags to handle the switch to `cargo doc`:
- `--no-deps` ensures we only build packages in the workspace.
- `--enable-index-page` (unstable) adds a landing page showing the list
of rendered crate docs.