site: add rel link to markdown output

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

layouts/partials/head.html

@@ -1,6 +1,9 @@
 <meta charset="utf-8" />
 <meta name="viewport" content="width=device-width, initial-scale=1" />
 {{ partial "meta.html" . }}
+{{- range .AlternativeOutputFormats -}}
+  <link rel="{{ .Rel }}" type="{{ .MediaType.Type }}" href="{{ .Permalink }}" />
+{{ end -}}
 {{ partial "utils/css.html" "-" }}
 {{- if hugo.IsProduction -}}
   <script