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

apps/docs/content/(array)/150-evaluate-reverse-polish-notation.mdx

@@ -28,6 +28,21 @@ sidebar:
 - `1 <= tokens.length <= 10^4`
 - `tokens[i]` is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["evalRPN(tokens)"]) --> I["stack = [], operations = + - * /"]
+  I --> L{"more token c?"}
+  L -- no --> E(["return stack[0]"])
+  L -- yes --> Q{"c is an operator?"}
+  Q -- no --> N["stack.append(int(c))"]
+  N --> L
+  Q -- yes --> P["y = stack.pop(), x = stack.pop() — order matters"]
+  P --> C["stack.append(operations[c](x, y))"]
+  C --> L
+```
+
 ## Solution
 
 ```py
@@ -53,3 +68,7 @@ class Solution:
 
         return stack[0]
 ```
+
+## Explanation
+
+<YouTube id="iu0082c4HDE" title="Evaluate Reverse Polish Notation - Leetcode 150 - Python" />