add support for OpenSearch to rustdoc

[lolbinarycat]

this would mainly be useful for searching the standard library directly from the browser search bar without waiting for the site to load first, but it could also be used for common crates like serde, or multi-crate documentation pages (eg. the nightly-rustc documentation)

[workingjubilee]

Duplicate with #24899 which describes why this is probably not happening: our "search engine" is client-side JS and an index delivered on page load.