rustdoc: add visible focus outline to rustdoc-toggle

The change in opacity is inconsistent with most of rustdoc, which uses
default browser styles for the focus outline. Unfortunately, just using
the default focus outline here won't work, because it gets applied to
the summary itself instead of the pseudo-element "real button."

Author: notriddle

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

@@ -1585,6 +1585,14 @@ details.rustdoc-toggle > summary:hover::before {
 	opacity: 1;
 }
 
+details.rustdoc-toggle > summary:focus-visible::before {
+	/* The SVG is black, and gets turned white using a filter.
+	   Do the same with the outline.
+	*/
+	outline: 1px dotted #000;
+	outline-offset: 1px;
+}
+
 details.rustdoc-toggle.top-doc > summary,
 details.rustdoc-toggle.top-doc > summary::before,
 details.rustdoc-toggle.non-exhaustive > summary,