Add noindex meta tag to prevent idxing by search engines.

Use templating to inject a meta tag in the html header to prevent
indexing by search engines.

Note: to be removed when URL redirection issue is solved.

Ref: https://developers.google.com/search/reference/robots_meta_tag

Author: rossbar

site/_templates/layout.html

@@ -0,0 +1,5 @@
+{% extends "!layout.html" %}
+{% block extrahead %}
+  <meta name="robots" content="noindex" />
+  {{ super() }}
+{% endblock %}