rustdoc: Submit examples to play.rust-lang.org

This grows a new option inside of rustdoc to add the ability to submit examples
to an external website. If the `--markdown-playground-url` command line option
or crate doc attribute `html_playground_url` is present, then examples will have
a button on hover to submit the code to the playground specified.

This commit enables submission of example code to play.rust-lang.org. The code
submitted is that which is tested by rustdoc, not necessarily the exact code
shown in the example.

Closes #14654

src/doc/rust.css

@@ -313,6 +313,19 @@ table th {
     padding: 5px;
 }
 
+/* Code snippets */
+
+.rusttest { display: none; }
+pre.rust { position: relative; }
+pre.rust a { transform: scaleX(-1); }
+.test-arrow {
+    display: inline-block;
+    position: absolute;
+    top: 0;
+    right: 10px;
+    font-size: 150%;
+}
+
 @media (min-width: 1170px) {
     pre {
         font-size: 15px;