docs: document how to include custom resources

exampleSite/content/en/usage/customization.md

@@ -0,0 +1,16 @@
+---
+title: Customization
+---
+
+{{< toc >}}
+
+## Custom resources
+
+To add custom resources to your site e.g. CSS or JavaScript files, create the `layouts/partials/head/custom.html` in the root dir of your project. Add the source files you want to include in the `static/` folder of your project, for example `static/css/custom.css` and `static/js/custom.js`. To include the files in your site, add the following code to `layouts/partials/head/custom.html`:
+
+```html
+<link rel="stylesheet" type="text/css" href="/css/custom.css" />
+<script type="text/javascript" src="/js/custom.js"></script>
+```
+
+Ensure to use the absolute path to the files.

layouts/partials/head/custom.html

@@ -1 +1,9 @@
-<!-- You can add custom elements to the page header here. -->
+<!--
+You can add custom elements to the page header here.
+
+Example:
+Ensure to use the absolute path to custom files and place the source files in the `static` folder.
+
+<link rel="stylesheet" type="text/css" href="/custom.css">
+<script type="text/javascript" src="/custom.js"></script>
+-->