Documentation: Add canonical relation to HTML output

[SamWhited]

While searching for documentation I've come across out of date old info listed alongside (or in front of) the current stable documentation ("the Python problem") in search results. It would be nice if all generated documentation for rust could include a meta tag telling search engines that the version of the docs it should index is the stable version:

<link href="https://doc.rust-lang.org/nightly/<path>.html" rel="canonical">

See also: https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Choosing_between_www_and_non-www_URLs#Using_%3Clink_relcanonical%3E

[steveklabnik]

This is a duplicate of #9461. Thank you for filing, though! Adding some of these links on that issue would be helpful. I'm going to close it so that we have one canonical (:wink:) issue for this.