feat(banner): add top banner (#777)

* feat: add top banner

* chore: use inside the example site

* chore: generate

* fix: banner with the burger navbar

* fix: compute the banner height to allow mutliple lines

* chore: better p style

layouts/_partials/head.html

@@ -77,6 +77,15 @@
 
   </script>
 
+  <script>
+    // The section must not be in the banner.js file because it can create a quick flash.
+
+    if (localStorage.getItem('{{ site.Params.banner.key | default `banner-closed` }}')) {
+      document.documentElement.style.setProperty("--hextra-banner-height", "0px");
+      document.documentElement.classList.add("hextra-banner-hidden");
+    }
+  </script>
+
   <!-- Math engine -->
   {{ $noop := .WordCount -}}
   {{- $engine := site.Params.math.engine | default "katex" -}}