feat: document new sitemap chunks feature

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

@@ -395,7 +395,59 @@ export default defineConfig({
 });
 ```
 
-### `i18n`
+### `chunks`
+
+<p>
+
+**Type:** `Record<string, (item: SitemapItem) => SitemapItem | undefined>`
+<Since v="3.7.0" pkg="@astrojs/sitemap" />
+</p>
+
+A map of functions that allows you to split your sitemap into multiple files based on custom logic. Each key in the object becomes the name of a separate sitemap file, and its corresponding function determines which URLs will be included in that chunk. This can be useful for instance if a specific section of your website changes very often and you'd like to specify a different change frequency for its entries.
+
+Each chunk function receives a `SitemapItem` and for each item returns either:
+* the modified `SitemapItem`, if the URL should be included in this chunk
+* `undefined`, if the URL should not be included in this chunk
+
+The example below shows how to split URLs into different sitemap files based on their path:
+
+```js title="astro.config.mjs" ins={8-26}
+import { defineConfig } from 'astro/config';
+import sitemap from '@astrojs/sitemap';
+
+export default defineConfig({
+  site: 'https://example.com',
+  integrations: [
+    sitemap({
+      chunks: {
+        'blog': (item) => {
+          if (/blog/.test(item.url)) {
+            item.changefreq = 'weekly';
+            item.lastmod = new Date();
+            item.priority = 0.9;
+            return item;
+          }
+        },
+        'glossary': (item) => {
+          if (/glossary/.test(item.url)) {
+            item.changefreq = 'monthly';
+            item.lastmod = new Date();
+            item.priority = 0.7;
+            return item;
+          }
+        }
+      },
+    }),
+  ],
+});
+```
+
+This configuration will generate the following files:
+
+- `sitemap-blog-0.xml`
+- `sitemap-glossary-0.xml`
+
+URLs that don't match any chunk will be placed in a default `sitemap-pages-0.xml` file.
 
 <p>