docs(docs): add Approach and Explanation sections with mermaid diagrams to problem pages

apps/docs/content/(array)/238-product-of-array-except-self.mdx

@@ -27,6 +27,17 @@ Can you solve the problem in O(1) extra space complexity? (The output array does
 - `-30 <= nums[i] <= 30`
 - The input is generated such that `answer[i]` is guaranteed to fit in a 32-bit integer.
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["productExceptSelf(nums)"]) --> I["answer = ones(n), rightArr = ones(n)"]
+  I --> P1["Pass 1 — left to right: answer[i] = nums[i-1] * answer[i-1]"]
+  P1 --> P2["Pass 2 — right to left: rightArr[i] = nums[i+1] * rightArr[i+1]"]
+  P2 --> P3["Pass 3: answer[i] *= rightArr[i]"]
+  P3 --> E(["return answer — no division needed"])
+```
+
 ## Solution
 
 ```js
@@ -51,3 +62,7 @@ var productExceptSelf = function(nums) {
   return answer;
 };
 ```
+
+## Explanation
+
+<YouTube id="bNvIQI2wAjk" title="Product of Array Except Self - Leetcode 238 - Python" />