add support for page summary and summary cards

lib/main.typ

@@ -1,12 +1,13 @@
 #import "./html_head.typ": html_head
 #import "./html_body.typ": html_body
 #import "./html_utils.typ": html_show
+#import "./summary.typ": summary, card-list
 
 
 /// Mail template function
 #let webpage(
   /// Page url
-  url,
+  url: none,
   /// Title of the page, default to document.title
   title: none,
   /// Use only for html 'lang' attribute.
@@ -41,6 +42,7 @@
   /// Body of the page
   body
 ) = {
+  assert(type(url) == str, message: "A page must have an url")
   show: html_show
   html.html(lang: lang, {
     html_head(