docs: add continuity manifesto and improve social metadata

Author: asboyer

_includes/metadata.liquid

@@ -49,24 +49,44 @@
     {% endunless %}
 {% endif %}
 
+{% if page.description %}
+    {% assign meta_description = page.description | strip_html | strip_newlines | normalize_whitespace %}
+{% elsif page.excerpt %}
+    {% assign meta_description = page.excerpt | strip_html | strip_newlines | normalize_whitespace %}
+{% else %}
+    {% assign meta_description = site.description | strip_html | strip_newlines | normalize_whitespace %}
+{% endif %}
+
+{% if page.og_image %}
+    {% assign social_image = page.og_image | absolute_url %}
+{% elsif site.og_image %}
+    {% assign social_image = site.og_image | absolute_url %}
+{% endif %}
+
+{% if page.title and page.url != '/' %}
+    {% capture social_title %}{{ page.title }} | {{ title }}{% endcapture %}
+{% else %}
+    {% assign social_title = title %}
+{% endif %}
+
 {% if site.serve_og_meta %}
     <!-- OpenGraph -->
-    <meta property="og:site_name" content="{{ title }}">
+    <meta property="og:site_name" content="{{ site.title }}">
     <meta property="og:type" content="{% if is_blog_post %}article{% else %}website{% endif %}">
-    <meta property="og:title" content="{% if page.title %}{{ title }} | {{ page.title }}{% else %}{{ title }}{% endif %}">
+    <meta property="og:title" content="{{ social_title }}">
     <meta property="og:url" content="{{ page.url | prepend: site.baseurl | prepend: site.url | remove_first: 'index.html' }}">
-    <meta property="og:description" content="{% if page.description %}{{ page.description }}{% else %}{{ site.description }}{% endif %}">
-    {% if page.og_image or site.og_image %}
-        <meta property="og:image" content="{% if page.og_image %}{{ page.og_image }}{% else %}{{ site.og_image }}{% endif %}">
+    <meta property="og:description" content="{{ meta_description }}">
+    {% if social_image %}
+        <meta property="og:image" content="{{ social_image }}">
     {% endif %}
     <meta property="og:locale" content="{{ site.lang }}">
 
     <!-- Twitter card -->
-    <meta name="twitter:card" content="summary">
-    <meta name="twitter:title" content="{% if page.title %}{{ page.title }}{% else %}{{ title }}{% endif %}">
-    <meta name="twitter:description" content="{% if page.description %}{{ page.description }}{% else %}{{ site.description }}{% endif %}">
-    {% if page.og_image or site.og_image %}
-        <meta name="twitter:image" content="{% if page.og_image %}{{ page.og_image }}{% else %}{{ site.og_image }}{% endif %}">
+    <meta name="twitter:card" content="{% if social_image %}summary_large_image{% else %}summary{% endif %}">
+    <meta name="twitter:title" content="{{ social_title }}">
+    <meta name="twitter:description" content="{{ meta_description }}">
+    {% if social_image %}
+        <meta name="twitter:image" content="{{ social_image }}">
     {% endif %}
     {% if site.x_username %}
         <meta name="twitter:site" content="@{{ site.x_username }}">