feat: make post description customizable (#1602)

---------

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

_includes/post-description.html

@@ -0,0 +1,16 @@
+{%- comment -%}
+  Get post description or generate it from the post content.
+{%- endcomment -%}
+
+{%- assign max_length = include.max_length | default: 200 -%}
+
+{%- capture description -%}
+{%- if post.description -%}
+  {{- post.description -}}
+{%- else -%}
+  {%- include no-linenos.html content=post.content -%}
+  {{- content | markdownify | strip_html -}}
+{%- endif -%}
+{%- endcapture -%}
+
+{{- description | strip | truncate: max_length | escape -}}

_includes/related-posts.html

@@ -81,10 +81,7 @@
               {% include datetime.html date=post.date lang=include.lang %}
               <h4 class="pt-0 my-2">{{ post.title }}</h4>
               <div class="text-muted">
-                <p>
-                  {% include no-linenos.html content=post.content %}
-                  {{ content | markdownify | strip_html | truncate: 200 | escape }}
-                </p>
+                <p>{% include post-description.html %}</p>
               </div>
             </div>
           </a>