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

apps/docs/content/(array)/1-two-sum.mdx

@@ -32,6 +32,20 @@ Can you come up with an algorithm that is less than O(n^2) time complexity?
 - `-10^9 <= target <= 10^9`
 - Only one valid answer exists.
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["twoSum(nums, target)"]) --> I["seen = {} — value to index"]
+  I --> L{"more (i, num) in nums?"}
+  L -- no --> E(["return []"])
+  L -- yes --> C["complement = target - num"]
+  C --> H{"complement in seen?"}
+  H -- yes --> R(["return [seen[complement], i]"])
+  H -- no --> W["seen[num] = i"]
+  W --> L
+```
+
 ## Solution
 
 ```py
@@ -45,3 +59,7 @@ class Solution:
             seen[num] = i
         return []
 ```
+
+## Explanation
+
+<YouTube id="KLlXCFG5TnA" title="Two Sum - Leetcode 1 - HashMap - Python" />