Add a stylesheet feature to the sitemap integration

[sarajw]

"Would love to add a stylesheet feature to the sitemap integration, similar to the styling features available for the RSS feed. Being able to pass in a custom XSL stylesheet with maybe a sensible default would be tremendous!"

Originally posted by @johneatmon in #146 (comment)

Sara: What he said!

I'm currently manually copying and pasting to make this happen, but that means the sitemap I will link to won't update on every build.

[florian-lefebvre]

This is now possible I believe: https://docs.astro.build/en/guides/integrations-guide/sitemap/#xslurl