docs: migrate to the built-in sitemap template

[XhmikosR]

Description

Motivation & Context

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-39951--twbs-bootstrap.netlify.app/sitemap.xml

Related issues

[julien-deramond]

LGTM! I've compared the generated sitemap.xml from main and this branch.
All the URLs are the same, meaning that the disable attribute works well.

The only differences are in the formatting and are not at all important; things like:

-  </url><url>
+  </url>
+  <url>

and

-<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
-xmlns:xhtml="http://www.w3.org/1999/xhtml">
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">