docs(sidebar): reorder data-structure entries in VuePress sidebar

- move `tree` before `graph` and `heap` in the data-structure children list
- align sidebar topic order to a clearer progression: linear structure -> tree -> graph -> heap

Author: paigeman

docs/.vuepress/sidebar/index.ts

@@ -222,9 +222,9 @@ export default sidebar({
           collapsible: true,
           children: [
             "linear-data-structure",
+            "tree",
             "graph",
             "heap",
-            "tree",
             "red-black-tree",
             "bloom-filter",
           ],