website: Large, on-brand link preview image for marketing pages (/intro/*)

Although /intro/getting-started includes docs content, those pages currently
redirect to the Learn platform, and so shouldn't be affected by the large unfurl
image.

website/layouts/intro.erb

@@ -1,4 +1,9 @@
 <% wrap_layout :inner do %>
+  <% content_for(:custom_share) do %>
+    <meta name="twitter:card" content="summary_large_image" />
+    <meta property="og:image" content="<%= image_url("og-image-large.png") %>"/>
+  <% end %>
+
   <% content_for :sidebar do %>
     <h4><a href="/intro/index.html">Introduction to Terraform</a></h4>