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

apps/docs/content/(array)/739-daily-temperatures.mdx

@@ -25,6 +25,20 @@ sidebar:
 - `1 <= temperatures.length <= 10^5`
 - `30 <= temperatures[i] <= 100`
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["dailyTemperatures(temperatures)"]) --> I["ans = zeros(n), stack = [] — indices of days still waiting"]
+  I --> L{"more (i, temp)?"}
+  L -- no --> E(["return ans"])
+  L -- yes --> W{"stack non-empty and temperatures[stack[-1]] < temp?"}
+  W -- yes --> P["prev = stack.pop(); ans[prev] = i - prev — today resolves that day"]
+  P --> W
+  W -- no --> A["stack.append(i)"]
+  A --> L
+```
+
 ## Solution
 
 ```py
@@ -39,3 +53,7 @@ class Solution:
             stack.append(i)
         return ans
 ```
+
+## Explanation
+
+<YouTube id="cTBiBSnjO3c" title="Daily Temperatures - Monotonic Stack - Leetcode 739 - Python" />