Add DocForge documentation site

docs/content/customization/plugins.md

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+# Writing Plugins
+
+Create JavaScript modules in the `plugins/` directory. Export hook functions such as `onParseMarkdown` or `onPageRendered` to modify the build.
+
+Example:
+
+```js
+module.exports = {
+  onPageRendered: async ({ html }) => {
+    return { html: html + '\n<!-- custom -->' };
+  }
+};
+```