Add .nojekyll to site root to prevent Jekyll processing

GitHub Pages was running Jekyll on the raw markdown sources instead
of serving the pre-built mdbook output. Adding .nojekyll to the
output root disables Jekyll processing.

xtask/src/main.rs

@@ -161,6 +161,9 @@ fn build_to(dir_name: &str) {
     println!("\n  {ok}/{} books built", BOOKS.len());
 
     write_landing_page(&out);
+
+    // Prevent GitHub Pages from processing the output with Jekyll
+    fs::write(out.join(".nojekyll"), "").expect("failed to create .nojekyll");
     println!("\nDone! Output in {dir_name}/");
 }