Add social cards support

Author: dbader

docs/_templates/layout.html

@@ -15,16 +15,28 @@
 
   {% if pagename == 'index' %}
   <link rel="canonical" href="https://docs.python-guide.org/"/>
+  <meta property="og:url" content="https://docs.python-guide.org/">
   {% elif pagename == '404' %}
   {# No canonical on our 404 template. #}
   {% else %}
   <link rel="canonical" href="https://docs.python-guide.org/{{ pagename }}/"/>
+  <meta property="og:url" content="https://docs.python-guide.org/{{ pagename }}">
   {% endif %}
 
   <link rel="icon" type="image/png" href="https://media.readthedocs.org/images/favicon.png">
 
   <meta name="google-site-verification" content="013PxE2_8KX9jdUSC5gr8QsfdxTXr1mFgmD9zplp5II" />
 
+  <meta name="twitter:card" content="summary">
+  <meta property="twitter:image" content="https://docs.python-guide.org/_static/python-guide-logo.png">
+  <meta property="og:image" content="https://docs.python-guide.org/_static/python-guide-logo.png">
+  <meta property="og:title" content="{{ title }}{{ titlesuffix }}">
+  <meta property="og:type" content="article">
+  {%- if metatags is defined %}
+  {# FIXME: For some reason the `meta` dict is always empty. Extract the desc from the `metatags` text.  #}
+  <meta property="og:description" content="{{ metatags[15:-24] }}">
+  {%- endif %}
+
   <script>window.rp_prop_id = '29182759436';</script>
   <script src="https://srv.realpython.net/tag.js" async></script>