refactor templates layout

@see https://gohugo.io/templates/new-templatesystem-overview/

layouts/_partials/single/main-header/series.html

@@ -0,0 +1,18 @@
+{{- /*
+  Renders the series of an item.
+
+  @params series    A list of series' for the item.
+  */
+-}}
+
+
+<p class="article-header__series">
+  Part of series:
+  {{ range .series }}
+    <a
+      class="u-url"
+      href="{{ .RelPermalink }}"
+      >{{ .LinkTitle }}</a
+    >
+  {{ end }}
+</p>