_content/site.tmpl: add canonical link

This change adds to each page a 'canonical' link
so that search engines will index only the go.dev
copy of a particular page instead of the tip.golang.org
one.

(Beware that in local testing the golangorg.hostPathHandler
function rewrites https://go.dev URLs to refer to localhost,
so this change does not have the desired effect in that mode.)

Updates golang/go#69600

Change-Id: I5bd8b0e0ac1b1d1a0d1b67b30e600246ed0beab8
Reviewed-on: https://go-review.googlesource.com/c/website/+/712040
Reviewed-by: Sean Liao <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: adonovan

_content/site.tmpl

@@ -13,6 +13,7 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <meta name="theme-color" content="#00add8">
+<link rel="canonical" href="https://go.dev{{.URL}}">
 <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Material+Icons">
 <link rel="stylesheet" href="/css/styles.css">
 <link rel="icon" href="/images/favicon-gopher.png" sizes="any">