Add link to channel layout docs.
There's a significant amount of information about the channel manifests in the documentation but the source doesn't currently link to it.
//! Installers use this info to customize Rust installations.
//!
//! See tests/channel-rust-nightly-example.toml for an example.
+//!
+//! Docs: https://forge.rust-lang.org/infra/channel-layout.html
use crate::errors::*;
use crate::utils::toml_utils::*;
