rustdoc search: prefer stable items in search results
fixes https://github.com/rust-lang/rust/issues/138067
this does add a new field to the search index, but since we're only listing unstable items instead of adding a boolean flag to every item, it should only increase the search index size of sysroot crates, since those are the only ones using the `staged_api` feature, at least as far as the rust project is concerned.