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

apps/docs/content/(tree)/98-validate-binary-search-tree.mdx

@@ -22,6 +22,23 @@ sidebar:
 - The number of nodes in the tree is in the range [1, 10^4].
 - `-2^31 <= Node.val <= 2^31 - 1`
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["isValidBST(root)"]) --> C["valid(root, float('-inf'), float('inf')) — the root may hold any value"]
+  C --> E(["return that result"])
+  subgraph valid["valid(node, low, high)"]
+    V1{"not node?"}
+    V1 -- yes --> V2(["return True"])
+    V1 -- no --> V3{"not (low < node.val < high)?"}
+    V3 -- yes --> V4(["return False — node breaks a bound set by an ancestor"])
+    V3 -- no --> V5["valid(node.left, low, node.val) and valid(node.right, node.val, high) — each child tightens one bound"]
+    V5 --> V6(["return that result"])
+  end
+  C --> V1
+```
+
 ## Solution
 
 ```py
@@ -44,3 +61,7 @@ class Solution:
 
         return valid(root, float("-inf"), float("inf"))
 ```
+
+## Explanation
+
+<YouTube id="s6ATEkipzow" title="Validate Binary Search Tree - Depth First Search - Leetcode 98" />