fix: remove rustdoc from rust-toolchain components

rustdoc is now bundled with the Rust toolchain and is no longer
available as a separately installable component in newer stable
channels, causing cargo deny CI to fail.

Assisted-by: Zed (Claude Sonnet 4.6)

Author: jdc-pub

rust-toolchain.toml

@@ -1,3 +1,3 @@
 [toolchain]
 channel = "stable"
-components = ["clippy", "rustfmt", "rustdoc"]
+components = ["clippy", "rustfmt"]