Update dependency mdbook to v0.4.48

// This program demonstrates the changes introduced in mdbook v0.4.48.
// It prints a short summary of the new features.
// The program compiles and runs on Rust 1.98.1 (edition 2024).

fn main() {
    // Footnotes now have back‑reference links and are rendered only at the bottom.
    println!("Footnotes: back‑reference links, bottom‑of‑page rendering");

    // Auto theme selection defaults to the system‑preferred mode and switches automatically.
    println!("Theme: auto selection based on system preference");

    // The search index file `searchindex.json` has been removed; only `searchindex.js` is generated.
    println!("Search index: only searchindex.js generated, searchindex.json removed");
}