Add documentation for xslURL in sitemap package

src/content/docs/en/guides/integrations-guide/sitemap.mdx

@@ -401,6 +401,26 @@ The resulting sitemap looks like this:
 ...
 ```
 
+### xslURL
+
+The URL of an XSL stylesheet to style and prettify your sitemap. 
+
+The value set can be either a path relative to your configured `site` URL for a local stylesheet, or can be an absolute URL link to an external stylesheet.
+
+```js title="astro.config.mjs" ins={8}
+import { defineConfig } from 'astro/config';
+import sitemap from '@astrojs/sitemap';
+
+export default defineConfig({
+  site: 'https://example.com',
+  integrations: [
+    sitemap({
+      xslURL: '/sitemap.xsl'
+    }),
+  ],
+});
+```
+
 ## Examples
 
 * The official Astro website uses Astro Sitemap to generate [its sitemap](https://astro.build/sitemap-index.xml).