Rollup merge of rust-lang#103554 - notriddle:notriddle/summary-focus-visible, r=jsha

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."

Preview: https://notriddle.com/notriddle-rustdoc-demos/focus-outline/test_dingus/fn.test.html

## Screenshots

### light

![image](https://user-images.githubusercontent.com/1593513/197903818-61ce09e2-024e-4ca9-9aba-764039561d0a.png)

### dark

![image](https://user-images.githubusercontent.com/1593513/197903765-76e428ea-71e8-4724-ad21-7a4f9d923ea2.png)

### ayu

![image](https://user-images.githubusercontent.com/1593513/197903790-322e22ae-64b5-4138-b931-9ff3ff54f3b0.png)

Author: Dylan-DPC

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

@@ -1585,6 +1585,15 @@ 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 in the dark themes.
+	   Do the same with the outline.
+	   The dotted 1px style is copied from Firefox's focus ring style.
+	*/
+	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,