feat(app): add sitemap file

sitemap.njk

@@ -0,0 +1,14 @@
+---
+permalink: '/sitemap.xml'
+layout: 'empty'
+eleventyExcludeFromCollections: true
+---
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  {% for page in collections.all %}
+  <url>
+    <loc>{{ page.url | url }}</loc>
+    <lastmod>{{ page.date.toISOString() }}</lastmod>
+  </url>
+  {% endfor %}
+</urlset>