Replace topbar dropdown menus working with JS by `<details>`#1939

GuillaumeGomez wants to merge 2 commits intorust-lang:masterrust-lang/docs.rs:masterfrom

GuillaumeGomez:simplify-topbar-drowpdown-menusGuillaumeGomez/docs.rs:simplify-topbar-drowpdown-menusCopy head branch name to clipboard