e5bbbca33e
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