feat: add site-wide social preview image settings (#1463)

- Add configuration field `social_preview_image` that sets the site-wide default social preview image.  

   For pages that do not have `page.image` set in font matter, the seo tag will use the image specified by `site.social_preview_image` as the Open Graph image.

- Refactored the generation of image URLs to reduce redundant code and enhance fault tolerance for missing or repeated slash `/` when defining image paths.

_includes/sidebar.html

@@ -3,18 +3,12 @@
 <aside aria-label="Sidebar" id="sidebar" class="d-flex flex-column align-items-end">
   <header class="profile-wrapper">
     <a href="{{ '/' | relative_url }}" id="avatar" class="rounded-circle">
-      {% if site.avatar != empty and site.avatar %}
-        {% capture avatar_url %}
-            {% if site.avatar contains '://' %}
-              {{ site.avatar }}
-            {% elsif site.img_cdn != empty and site.img_cdn %}
-              {{ site.avatar | prepend: site.img_cdn }}
-            {% else %}
-              {{ site.avatar | relative_url }}
-            {% endif %}
-          {% endcapture %}
-        <img src="{{ avatar_url | strip }}" width="112" height="112" alt="avatar" onerror="this.style.display='none'">
-      {% endif %}
+      {%- if site.avatar != empty and site.avatar -%}
+        {%- capture avatar_url -%}
+          {% include img-url.html src=site.avatar %}
+        {%- endcapture -%}
+        <img src="{{- avatar_url -}}" width="112" height="112" alt="avatar" onerror="this.style.display='none'">
+      {%- endif -%}
     </a>
 
     <h1 class="site-title">