Add a viewport meta tag in simple theme for better mobile support

See MDN article about this:

https://developer.mozilla.org/en-US/docs/Web/HTML/Viewport_meta_tag

pelican/themes/simple/templates/base.html

@@ -5,6 +5,7 @@
         <title>{% block title %}{{ SITENAME }}{% endblock title %}</title>
         <meta charset="utf-8" />
         <meta name="generator" content="Pelican" />
+        <meta name="viewport" content="width=device-width, initial-scale=1.0">
         {% if FEED_ALL_ATOM %}
         <link href="{{ FEED_DOMAIN }}/{% if FEED_ALL_ATOM_URL %}{{ FEED_ALL_ATOM_URL }}{% else %}{{ FEED_ALL_ATOM }}{% endif %}" type="application/atom+xml" rel="alternate" title="{{ SITENAME }} Full Atom Feed" />
         {% endif %}