docs(docs): add Approach diagrams and Explanation sections to LeetCode problem documentation

apps/docs/content/(tree)/102-binary-tree-level-order-traversal.mdx

@@ -25,6 +25,26 @@ sidebar:
 - The number of nodes in the tree is in the range [0, 2000].
 - `-1000 <= Node.val <= 1000`
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["levelOrder(root)"]) --> I["res = [], q = deque(), q.append(root)"]
+  I --> W{"q non-empty?"}
+  W -- no --> E(["return res"])
+  W -- yes --> L["qLen = len(q), level = [] — qLen freezes the current level's size"]
+  L --> F{"more i in range(qLen)?"}
+  F -- no --> V{"level non-empty?"}
+  V -- yes --> A["res.append(level)"]
+  A --> W
+  V -- no --> W
+  F -- yes --> P["node = q.popleft()"]
+  P --> N{"node is not None?"}
+  N -- no --> F
+  N -- yes --> C["level.append(node.val), q.append(node.left), q.append(node.right) — None children are filtered on the next level"]
+  C --> F
+```
+
 ## Solution
 
 ```py
@@ -59,3 +79,7 @@ class Solution:
 
         return res
 ```
+
+## Explanation
+
+<YouTube id="6ZnyEApgFYg" title="Binary Tree Level Order Traversal - BFS - Leetcode 102" />