Remove plugin docs and fix sidebar arrow alignment

docs/content/03-customization/plugins.md

@@ -1,13 +0,0 @@
-# Writing Plugins
-
-Place JavaScript files in the `plugins/` directory. Export hook functions such as `onParseMarkdown` or `onPageRendered` to modify the build process. Hooks receive data objects that you can transform before Archivox writes the final files.
-
-Example:
-
-```js
-module.exports = {
-  onPageRendered: async ({ html }) => {
-    return { html: html + '\n<!-- custom -->' };
-  }
-};
-```