Add ToC scroolbar for huge articles

Signed-off-by: GitHub Action <action@github.com>

assets/scss/_scrolltotop.scss

@@ -0,0 +1,32 @@
+.scroll-up {
+    .scroll {
+        #st-ring {
+            stroke: $stt-stroke;
+        }
+
+        #st-circle {
+            fill: $stt-circle;
+        }
+
+        #st-arrow {
+            fill: $stt-arrow;
+        }
+    }
+
+    position: fixed;
+    bottom: 10%;
+    right: 2%;
+    z-index: 1;
+    opacity: 0;
+    transition: all 0.5s ease;
+}
+
+.hide {
+    opacity: 0;
+    transform: translateY(20px);
+}
+
+.show {
+    opacity: 1;
+    transform: translateY(0);
+}