Fix playpen links to not all be the same

Fix playpen links to not all be the same

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.