Use <main> and <article> tags in simple theme

Add a <main> tag to surround all the content blocks, so that it's easier
to target the main part of a page (be it an article, the list of posts
or the different categories) using CSS.

Because of this, the <section> part of the article.html template is made
redundant, so it is removed.

Finally, the generic <div> is replaced by an <article> tag to surround
the article's content.

pelican/themes/simple/templates/base.html

@@ -51,8 +51,10 @@
           {% endfor %}
         {% endif %}
         </ul></nav><!-- /#menu -->
+        <main>
         {% block content %}
         {% endblock %}
+        </main>
         <footer id="contentinfo" class="body">
                 <address id="about" class="vcard body">
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