Add social cards support

Author: dbader

docs/_templates/layout.html

@@ -15,16 +15,28 @@
 
 {% if pagename == 'index' %}
 <linkrel="canonical" href="https://docs.python-guide.org/"/>
+<metaproperty="og:url" content="https://docs.python-guide.org/">
 {% elif pagename == '404' %}
 {# No canonical on our 404 template. #}
 {% else %}
 <linkrel="canonical" href="https://docs.python-guide.org/{{pagename }}/"/>
+<metaproperty="og:url" content="https://docs.python-guide.org/{{pagename }}">
 {% endif %}
 
 <linkrel="icon" type="image/png" href="https://media.readthedocs.org/images/favicon.png">
 
 <metaname="google-site-verification" content="013PxE2_8KX9jdUSC5gr8QsfdxTXr1mFgmD9zplp5II" />
 
+<metaname="twitter:card" content="summary">
+<metaproperty="twitter:image" content="https://docs.python-guide.org/_static/python-guide-logo.png">
+<metaproperty="og:image" content="https://docs.python-guide.org/_static/python-guide-logo.png">
+<metaproperty="og:title" content="{{title }}{{titlesuffix }}">
+<metaproperty="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. #}
+<metaproperty="og:description" content="{{metatags[15:-24] }}">
+{%- endif %}
+
 <script>window.rp_prop_id='29182759436';</script>
 <scriptsrc="https://srv.realpython.net/tag.js" async></script>