Wrap content in <main> tag in base layout

Wraps page content within a <main> element for improved semantic HTML structure and accessibility ([document should have one main landmark
](https://dequeuniversity.com/rules/axe/4.10/landmark-one-main)).

Author: griffenedge

_layouts/base.html

@@ -119,14 +119,16 @@ <h1><a href="{{ site.baseurl }}/">{{ site.title }}</a></h1>
         {% endif -%}
         </header>
 
-      {% comment -%}
-        This is primarily intended to remove the trailing slash from the
-        self-closing `img` tags that result from rendered Markdown text.
-      {% endcomment -%}
-      {% capture rendered_content -%}
-        {{ content }}
-      {% endcapture -%}
-      {{ rendered_content | replace: " />", ">" | replace: "/>", ">" -}}
+      <main> 
+        {% comment -%}
+          This is primarily intended to remove the trailing slash from the
+          self-closing `img` tags that result from rendered Markdown text.
+        {% endcomment -%}
+        {% capture rendered_content -%}
+          {{ content }}
+        {% endcapture -%}
+        {{ rendered_content | replace: " />", ">" | replace: "/>", ">" -}}
+      </main>
     </div>
 
     {% if site.forkme_nwo -%}