docs: add notes for sitemap + base

closes #3863

Author: brc-dd

docs/guide/sitemap-generation.md

@@ -3,13 +3,11 @@
 VitePress comes with out-of-the-box support for generating a `sitemap.xml` file for your site. To enable it, add the following to your `.vitepress/config.js`:
 
 ```ts
-import { defineConfig } from 'vitepress'
-
-export default defineConfig({
+export default {
   sitemap: {
     hostname: 'https://example.com'
   }
-})
+}
 ```
 
 To have `<lastmod>` tags in your `sitemap.xml`, you can enable the [`lastUpdated`](../reference/default-theme-last-updated) option.
@@ -19,24 +17,31 @@ To have `<lastmod>` tags in your `sitemap.xml`, you can enable the [`lastUpdated
 Sitemap support is powered by the [`sitemap`](https://www.npmjs.com/package/sitemap) module. You can pass any options supported by it to the `sitemap` option in your config file. These will be passed directly to the `SitemapStream` constructor. Refer to the [`sitemap` documentation](https://www.npmjs.com/package/sitemap#options-you-can-pass) for more details. Example:
 
 ```ts
-import { defineConfig } from 'vitepress'
-
-export default defineConfig({
+export default {
   sitemap: {
     hostname: 'https://example.com',
     lastmodDateOnly: false
   }
-})
+}
+```
+
+If you're using `base` in your config, you should append it to the `hostname` option:
+
+```ts
+export default {
+  base: '/my-site/',
+  sitemap: {
+    hostname: 'https://example.com/my-site/'
+  }
+}
 ```
 
 ## `transformItems` Hook
 
 You can use the `sitemap.transformItems` hook to modify the sitemap items before they are written to the `sitemap.xml` file. This hook is called with an array of sitemap items and expects an array of sitemap items to be returned. Example:
 
 ```ts
-import { defineConfig } from 'vitepress'
-
-export default defineConfig({
+export default {
   sitemap: {
     hostname: 'https://example.com',
     transformItems: (items) => {
@@ -49,5 +54,5 @@ export default defineConfig({
       return items
     }
   }
-})
+}
 ```