Document how to add custom html header content (#613)

Author: jarrodmillman

doc/content/user_guide/features.md

@@ -14,6 +14,10 @@ Blog post layouts:
 - `layouts/partials/posts/tag.html`: Tag page layout
 - `layers/partials/posts/comments.html`: Empty by default; can be overridden to place a comments section
 
+## Add custom HTML header content
+
+- `layouts/partials/head.html`
+
 ## Shortcut list
 
 The depths of the shortcut list on the left of each post can be

layouts/_default/baseof.html

@@ -11,7 +11,7 @@
     <!-- Add MathJax, if needed -->
     {{ partial "mathjax.html" }}
 
-    <!-- Render custom header -->
+    <!-- Render custom user header -->
     {{ partial "head.html" . -}}
 
     <!-- Refer: https://regisphilibert.com/blog/2018/08/hugo-multilingual-part-1-managing-content-translation/ -->

layouts/partials/head.html

@@ -0,0 +1 @@
+<!-- Leave this file empty. This is a placeholder for theme users to customize head content. -->