Add summaries to the atom feeds

src/feeds/atom.njk

@@ -24,6 +24,7 @@
     <updated>{{ post.date | dateToRfc3339 }}</updated>
     <id>{{ absolutePostUrl }}</id>
     <content xml:lang="{{ metadata.site.language }}" type="html">{{ post.templateContent | htmlToAbsoluteUrls(absolutePostUrl) }}</content>
+    <summary>{{ post.content | excerpt }}</summary>
   </entry>
   {%- endfor %}
 </feed>