docs: Add information about dynamic static split server islands

src/content/docs/en/reference/integrations-reference.mdx

@@ -157,6 +157,44 @@ export default {
 }
 ```
 
+<p><Since v="5.1" /></p>
+
+##### Private Config Options
+
+This hook gives you the opportunity to add private config options that are exposed only to integrations.
+There are certain options that are not directly exposed to users, but are allowed to be written from integrations.
+
+##### `serverIslandDynamicBase`
+
+**Type:** `string`
+
+The base URL of the exported server islands. This option is not exposed to users,
+but can be written from integrations to allow an integration to change the base URL of the exported server islands.
+
+Example usage:
+
+```js
+export default {
+  hooks: {
+    'astro:config:setup': ({ updateConfig }) => {
+      updateConfig({
+        serverIslandDynamicBase: 'https://my-server-islands.com'
+      })
+    }
+  }
+}
+```
+
+Would result in the following client HTML output:
+
+```html
+<script>
+  //...
+  let response = await fetch('https://my-server-islands.com/_server-islands/...')
+</script>
+```
+
+
 #### `addRenderer` option
 
 **Type:** `(renderer:` [`AstroRenderer`](https://github.com/withastro/astro/blob/fdd607c5755034edf262e7b275732519328a33b2/packages/astro/src/%40types/astro.ts#L872-L883) `) => void;`