Fixes#20732, that all links in some modules point to the same code
examples was reported. The ID's generated for documents in
librustdoc are not all unique, which means the code rendered as
text is not being properly selected.
This change removes the unique id generation and instead changes the
frontend code to grab the correct code sample by it's relative
position in the dom.
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
