feat: make post description customizable (#1602)

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Co-authored-by: Cotes Chung <11371340+cotes2020@users.noreply.github.com>

_layouts/post.html

@@ -14,6 +14,9 @@ tail_includes:
 <article class="px-1">
   <header>
     <h1 data-toc-skip>{{ page.title }}</h1>
+    {% if page.description %}
+      <p class="post-desc fw-light mb-4">{{ page.description }}</p>
+    {% endif %}
 
     <div class="post-meta text-muted">
       <!-- published date -->