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/librustdoc/html/static/main.js

@@ -678,7 +678,7 @@
         window.register_implementors(window.pending_implementors);
     }
 
-    // See documentaiton in html/render.rs for what this is doing.
+    // See documentation in html/render.rs for what this is doing.
     var query = getQueryStringParams();
     if (query['gotosrc']) {
         window.location = $('#src-' + query['gotosrc']).attr('href');