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

apps/docs/content/(array)/1636-sort-array-by-increasing-frequency.mdx

@@ -27,6 +27,18 @@ sidebar:
 - `1 <= nums.length <= 100`
 - `-100 <= nums[i] <= 100`
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["frequencySort(nums)"]) --> F["freq: count every value in nums"]
+  F --> C["sort nums with a two-key comparator"]
+  C --> K1["primary: freq[a] - freq[b] — rarer first"]
+  C --> K2["tie-break: b - a — larger value first"]
+  K1 --> R(["return nums"])
+  K2 --> R
+```
+
 ## Solution
 
 ```js
@@ -40,3 +52,7 @@ var frequencySort = function (nums) {
   return nums.sort((a, b) => freq[a] - freq[b] || b - a);
 };
 ```
+
+## Explanation
+
+<YouTube id="Evq1SfUbhBg" title="Sort Array by Increasing Frequency - Leetcode 1636 - Python" />