scss: improve spacing of header navigation

- Improves the spacing of the header navigation item, in order to ease
  their use.

Co-Authored-by: iGor milhit <igor@milhit.ch>

assets/scss/_header.scss

@@ -31,5 +31,10 @@
 
     }
 
+    .header-navigation {
+      width: 100%;
+      justify-content: space-evenly;
+    }
+
   }
 }

layouts/partials/header.html

@@ -16,7 +16,7 @@
         </a>
       </li>
     </ul>
-    <ul>
+    <ul class="header-navigation">
       {{ $currentPage := . -}}
       {{ range sort .Site.Menus.main -}}
       <li class="nav-item secondary">