Function render_html 

pub fn render_html(
    item: &DocItem,
    file_stem: &str,
    siblings: &[(String, String)],
    index_link: &str,
) -> String

Render a single item page as a standalone HTML document.

siblings is a list of (slug, name) pairs for the sidebar — pass an empty slice for no sidebar. index_link controls the “back to file index” link target at the top of the page.