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

apps/docs/content/(array)/217-contains-duplicate.mdx

@@ -27,6 +27,19 @@ sidebar:
 - `1 <= nums.length <= 10^5`
 - `-10^9 <= nums[i] <= 10^9`
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["containsDuplicate(nums)"]) --> I["freq = {}"]
+  I --> L{"more n in nums?"}
+  L -- no --> E(["return false"])
+  L -- yes --> C["freq[n] = (freq[n] or 0) + 1"]
+  C --> Q{"freq[n] >= 2?"}
+  Q -- yes --> R(["return true"])
+  Q -- no --> L
+```
+
 ## Solution
 
 ```js
@@ -45,3 +58,7 @@ var containsDuplicate = function(nums) {
   return false;
 };
 ```
+
+## Explanation
+
+<YouTube id="3OamzN90kPg" title="Contains Duplicate - Leetcode 217 - Python" />