Function render_sitemap 

pub fn render_sitemap(
    cases: &[(String, String)],
    people: &[(String, String)],
    organizations: &[(String, String)],
    base_url: &str,
) -> String

Generate a sitemap XML string.

All tuples are (slug, display_name) where slug is the full file-path slug (e.g. cases/id/corruption/2024/hambalang-case).