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/article.html

@@ -22,7 +22,6 @@
 {% endblock %}
 
 {% block content %}
-<section id="content" class="body">
   <header>
     <h1 class="entry-title">
       <a href="{{ SITEURL }}/{{ article.url }}" rel="bookmark"
@@ -60,8 +59,7 @@
     </div>
     {% endif %}
   </footer><!-- /.post-info -->
-  <div class="entry-content">
+  <article>
     {{ article.content }}
-  </div><!-- /.entry-content -->
-</section>
+  </article>
 {% endblock %}

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">
                 Proudly powered by <a href="https://getpelican.com/">Pelican</a>,