Added heade component which includes the main navigation

src/_includes/components/header.njk

@@ -0,0 +1,12 @@
+{% set navItems = collections.all | eleventyNavigation %}
+<header role="banner">
+  <nav role="navigation" aria-label="main navigation" >
+    <ul>
+      {%- for item in navItems %}
+      <li>
+        <a href="{{ item.url }}">{{ item.key }}</a>
+      </li>
+      {%- endfor -%}
+    </ul>
+  </nav>
+</header>