demo: add utterances comments system

demoSite/layouts/partials/comments.html

@@ -0,0 +1,12 @@
+<div class="utterances">
+  <iframe class="utterances-frame"></iframe>
+</div>
+<script
+  src="https://utteranc.es/client.js"
+  repo="mirus-ua/hugo-theme-re-terminal"
+  issue-term="pathname"
+  label="Comment ✨💬✨"
+  theme="dark-blue"
+  crossorigin="anonymous"
+  async
+></script>