Add social card og:image (#219)

* docs: add social card og:image meta

* docs: use palette-optimized social card (~70% smaller)

Author: lesnik512

docs/assets/social-card.png

@@ -3,3 +3,24 @@
 {% block announce %}
   <strong>that-depends</strong>  4.0 just released! If you are upgrading, check out the <a href="{{config.repo}}/migration/v4/">migration guide</a>.
 {% endblock %}
+
+{% block extrahead %}
+  {{ super() }}
+  {% set base = config.site_url if config.site_url.endswith('/') else config.site_url ~ '/' %}
+  {% set card = base ~ 'assets/social-card.png' %}
+  {% set title = (page.title ~ ' · ' ~ config.site_name) if (page.title and not page.is_homepage) else config.site_name %}
+  {% set description = page.meta.description if page.meta and page.meta.description else config.site_description %}
+  <meta property="og:type" content="website">
+  <meta property="og:site_name" content="{{ config.site_name }}">
+  <meta property="og:title" content="{{ title }}">
+  <meta property="og:description" content="{{ description }}">
+  <meta property="og:url" content="{{ page.canonical_url }}">
+  <meta property="og:image" content="{{ card }}">
+  <meta property="og:image:type" content="image/png">
+  <meta property="og:image:width" content="1280">
+  <meta property="og:image:height" content="640">
+  <meta name="twitter:card" content="summary_large_image">
+  <meta name="twitter:title" content="{{ title }}">
+  <meta name="twitter:description" content="{{ description }}">
+  <meta name="twitter:image" content="{{ card }}">
+{% endblock %}