Add canonical URL (#514)

* Add canonical URL

---------

Co-authored-by: Markus Neteler <neteler@gmail.com>

Author: petrasovaa

themes/grass/layouts/partials/head.html

@@ -23,6 +23,7 @@
   <meta name="apple-mobile-web-app-capable" content="yes" />
   <meta name="apple-mobile-web-app-title" content="GRASS" />
   <meta name="apple-mobile-web-app-status-bar-style" content="default" />
+  <link rel="canonical" href="{{ .Permalink }}" />
   <link rel="stylesheet" href="{{ .Site.BaseURL }}plugins/bootstrap/bootstrap.min.css">
   <link rel="stylesheet" href="{{ .Site.BaseURL }}plugins/highlight/hybrid.css"> 
   <link rel="stylesheet" href="{{ .Site.BaseURL }}plugins/flag-icons-css/css/flag-icon.min.css">