Add shortcode `section` with params title, img, alt, credit

templates/shortcodes/section.html

+<section class="section">
+  <h2 class="title is-2">{{ title }}</h2>
+  <div class="columns is-vcentered">
+    <div class="column">
+      <figure class="image is-4by3">
+        {% set image = resize_image(path="@/" ~ page.colocated_path ~ "/" ~ img, width=800, height=600) -%}
+        <img src="{{image.url}}" alt="{{ alt }}"/>
+        {% if credit %}
+        <figcaption class="p-1 has-text-white has-background-black is-size-7 is-italic">{{ credit }}</figcaption>
+        {% endif %}
+      </figure>
+    </div>
+    <div class="column content">
+      {{ body|markdown|safe }}
+    </div>
+  </div>
+</section>