rustdoc: clean up the layout for annotated version numbers

This should result in a layout for the actual standard library,
when built on CI, that looks like this:

    _____
   /     \ std
   |  R  | 1.74.0-nightly
   \_____/

   (203c57dbe 2023-09-17)

Having the whole version as one string caused it to flex wrap,
because the sidebar isn't wide enough to fit the whole thing.

src/librustdoc/html/static/css/rustdoc.css

@@ -492,6 +492,7 @@ ul.block, .block li {
 }
 
 .sidebar-elems,
+.sidebar > .version,
 .sidebar > h2 {
 	padding-left: 24px;
 }