Rollup merge of #80316 - ehuss:rustdoc-index, r=Mark-Simulacrum

Include rustdoc in the compiler docs index.

This should help ensure that the index page at https://doc.rust-lang.org/nightly/nightly-rustc/ includes a link to the rustdoc docs as well.

Fixes #80307

src/bootstrap/doc.rs

@@ -628,6 +628,8 @@ impl Step for Rustdoc {
  cargo.arg("-p").arg("rustdoc");
 
  cargo.rustdocflag("--document-private-items");
+ cargo.rustdocflag("--enable-index-page");
+ cargo.rustdocflag("-Zunstable-options");
  builder.run(&mut cargo.into());
  }
 }