feat: add support for changefreq in sitemap

src/common/sitemap.njk

@@ -15,6 +15,7 @@ excludeFromSitemap: true
     <url>
         <loc>{{ meta.url }}{{ page.url }}</loc>
         <lastmod>{{ lastmod | toIsoString }}</lastmod>
+        <changefreq>{{ page.data.changeFreq if page.data.changeFreq else "monthly" }}</changefreq>
     </url>
     {% endif %}
   {% endfor %}