Add ARIA role to sidebar toggle in Rustdoc

This indicates that the div is an interactive element, and makes the
sidebar toggle "clickable" in assistive technologies.

Fixes rust-lang#82582

Author: tazjin

src/librustdoc/html/layout.rs

@@ -75,7 +75,7 @@ crate fn render<T: Print, S: Print>(
     <![endif]-->\
     {before_content}\
     <nav class=\"sidebar\">\
-        <div class=\"sidebar-menu\">&#9776;</div>\
+        <div class=\"sidebar-menu\" role=\"button\">&#9776;</div>\
         {logo}\
         {sidebar}\
     </nav>\