fix missing include

_includes/custom-head.html

@@ -0,0 +1,6 @@
+{% comment %}
+  Placeholder to allow defining custom head, in principle, you can add anything here, e.g. favicons:
+
+  1. Head over to https://realfavicongenerator.net/ to add your own favicons.
+  2. Customize default _includes/custom-head.html in your source directory and insert the given code snippet.
+{% endcomment %}