Add lastmod and ensure we also add DocumentationGroup navigation items

[Mpdreamz]

Preview:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<urlset xlmns="http://www.sitemaps.org/schemas/sitemap/0.9">
  <url>
    <loc>https://www.elastic.co/docs/get-started/introduction</loc>
    <lastmod>2025-04-15T07:16:37+00:00</lastmod>
  </url>
  <url>
    <loc>https://www.elastic.co/docs/get-started/the-stack</loc>
    <lastmod>2025-04-15T07:16:37+00:00</lastmod>
  </url>
  <url>
    <loc>https://www.elastic.co/docs/get-started/deployment-options</loc>
    <lastmod>2025-04-15T07:16:37+00:00</lastmod>
  </url>
  <url>
...

[cotti]

LGTM!