Add canonical link from versioned documentation pages

layouts/header.html

@@ -25,6 +25,7 @@
     <link rel="icon" type="image/png" href="/assets/favicons/favicon-96x96.png" sizes="96x96">
     <link rel="icon" type="image/png" href="/assets/favicons/favicon-16x16.png" sizes="16x16">
     <link rel="manifest" href="/assets/favicons/android-chrome-manifest.json">
+    <% if (c = @item[:repo_docs]) && c[:canonical] %><link rel="canonical" href="<%= @item.path.sub(c[:items_root], c[:canonical]) %>" /><% end %>
     <meta name="msapplication-TileColor" content="#da532c">
     <meta name="msapplication-TileImage" content="/assets/favicons/mstile-144x144.png">
     <meta name="theme-color" content="#ffffff">

nanoc.yaml

@@ -68,6 +68,7 @@ data_sources:
       name: '2.0'
       repository: https://github.com/prometheus/prometheus.git
       refspec: master
+      canonical: /docs/prometheus/latest/
   -
     type: repo_docs
     items_root: /docs/prometheus/latest/
@@ -82,6 +83,7 @@ data_sources:
       name: '1.8'
       repository: https://github.com/prometheus/prometheus.git
       refspec: release-1.8
+      canonical: /docs/prometheus/latest/
       outdated: /docs/prometheus/latest/
   -
     type: static