Function handle_crate_request_watch 

pub async fn handle_crate_request_watch<B>(
    req: Request<B>,
    static_: Static,
    crate_name: String,
    build_id: Arc<AtomicU64>,
) -> Result<Response<BoxBody<Bytes, Error>>, Error>

handle crate doc request in watch mode.

Responds to /_buildid with the current build counter so the injected JavaScript can detect when a rebuild has completed and reload the page. HTML responses have RELOAD_SCRIPT injected before </body>.