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

apps/docs/content/(array)/215-kth-largest-element-in-an-array.mdx

@@ -21,6 +21,20 @@ sidebar:
 - `1 <= k <= nums.length <= 10^5`
 - `-10^4 <= nums[i] <= 10^4`
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["findKthLargest(nums, k)"]) --> F{"more i in range(len(nums))?"}
+  F -- yes --> N["nums[i] = -nums[i] — negate so the min-heap acts as a max-heap"]
+  N --> F
+  F -- no --> H["heapq.heapify(nums)"]
+  H --> W{"more _ in range(k - 1)?"}
+  W -- yes --> P["heapq.heappop(nums) — discard the k-1 largest"]
+  P --> W
+  W -- no --> E(["return -heapq.heappop(nums)"])
+```
+
 ## Solution
 
 ```py
@@ -39,3 +53,7 @@ class Solution:
 
         return -heapq.heappop(nums)
 ```
+
+## Explanation
+
+<YouTube id="XEmy13g1Qxc" title="Kth Largest Element in an Array - Quick Select - Leetcode 215 - Python" />