Add sitemap.xml for /current and /mongocxx-3.11.0

Author: eramongodb

sitemap_index.xml

@@ -1,6 +1,16 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  <!-- Home Page. -->
   <sitemap>
     <loc>https://mongocxx.org/sitemap.xml</loc>
   </sitemap>
+  <!-- API Documentation Pages. -->
+  <sitemap>
+    <loc>https://mongocxx.org/api/current/sitemap.xml</loc>
+    <lastmod>2024-10-12</lastmod>
+  </sitemap>
+  <sitemap>
+    <loc>https://mongocxx.org/api/mongocxx-3.11.0/sitemap.xml</loc>
+    <lastmod>2024-10-12</lastmod>
+  </sitemap>
 </sitemapindex>