docs: added information on sitemap generation (#1145)

Co-authored-by: Sébastien Chopin <seb@nuxtjs.com>

docs/content/3.guide/3.recipes/1.sitemap.md

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+---
+title: Sitemap
+---
+
+# Sitemap Generation
+
+A sitemap file is useful for helping Google to better index your website, ensuring that the content you write can be visible on search results.
+
+This can be created utilising the `sitemap` library, so you'll need to install that which can be done like so:
+
+```bash
+yarn add --dev sitemap
+```
+
+## Server Route
+
+We will be utilising the [server routes](https://v3.nuxtjs.org/guide/features/server-routes) available within Nuxt, and to do so you'll need to create the `server/` directory within your websites root directly.
+
+Once this is done, create a `routes/` directory inside this, and add a `sitemap.xml.ts` file, this will translate to `/sitemap.xml`.
+
+You'll need to add the following:
+
+```ts [server/routes/sitemap.xml.ts]
+import { serverQueryContent } from '#content/server'
+import { SitemapStream, streamToPromise } from 'sitemap'
+
+export default defineEventHandler(async (event) => {
+  // Fetch all documents
+  const docs = await serverQueryContent(event).find()
+  const sitemap = new SitemapStream({
+    hostname: 'https://example.com'
+  })
+
+  for (const doc of docs) {
+    sitemap.write({
+      url: doc._path,
+      changefreq: 'monthly'
+    })
+  }
+  sitemap.end()
+
+  return streamToPromise(sitemap)
+})
+```
+
+Now, once users go to `https://example.com/sitemap.xml`, you'll find the generated XML file with all your pages.
+
+When using `nuxt generate`, you may want to pre-render the sitemap since the server route won't be able to run on a static hosting.
+
+You can do this using the `nitro.prerender` option in your `nuxt.config`:
+
+```ts [nuxt.config.ts]
+import { defineNuxtConfig } from 'nuxt'
+
+export default defineNuxtConfig({
+  // ...
+  nitro: {
+    prerender: {
+      routes: ['/sitemap.xml']
+    }
+  }
+})
+```

docs/content/3.guide/3.recipes/_dir.yml

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+icon: heroicons-outline:book-open