feat: add index page to sitemap.xml (#1318)

src/main/resources/templates/common/web/sitemap_xml.ftl

 <?xml version="1.0" encoding="UTF-8"?>
 <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+    <url>
+        <loc>${blog_url!}</loc>
+        <lastmod>${options.birthday?number_to_datetime?iso_local}</lastmod>
+    </url>
     <#if posts?? && posts?size gt 0>
         <#list posts as post>
             <url>
@@ -28,4 +32,4 @@
             </#list>
         </#if>
     </@tagTag>
-</urlset>
\ No newline at end of file
+</urlset>