docs: added scroll-to-top button with JS and CSS

docs/_static/scrolltop.js

@@ -0,0 +1,17 @@
+window.addEventListener("scroll", function () {
+  const btn = document.getElementById("scrollToTop");
+  btn.style.display = window.scrollY > 300 ? "block" : "none";
+});
+
+function scrollToTop() {
+  window.scrollTo({ top: 0, behavior: "smooth" });
+}
+
+document.addEventListener("DOMContentLoaded", () => {
+  const btn = document.createElement("button");
+  btn.id = "scrollToTop";
+  btn.textContent = "↑ Top";
+  btn.setAttribute("aria-label", "Scroll to top");
+  btn.onclick = scrollToTop;
+  document.body.appendChild(btn);
+});