Fix link previews on Twitter (#3856)

Author: simonw

docs/custom_theme/main.html

@@ -7,7 +7,7 @@
 <!-- / Fathom -->
 
 
-<meta name="twitter:card" content="summary_large_image">
+<meta name="twitter:card" content="summary">
 <meta name="twitter:site" content="@textualizeio">
 <meta name="twitter:creator" content="@willmcgugan">
 <meta property="og:title" content="Textual - {{ page.title }}">