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

apps/docs/content/(string)/20-valid-parentheses.mdx

@@ -33,6 +33,24 @@ sidebar:
 - `1 <= s.length <= 10^4`
 - `s` consists of parentheses only '()[]{}'.
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["isValid(s)"]) --> I["stack = [], pairs = closer to opener"]
+  I --> L{"more char c?"}
+  L -- no --> Z{"stack empty?"}
+  Z -- yes --> T(["return True"])
+  Z -- no --> U(["return False — unclosed openers left"])
+  L -- yes --> Q{"c is a closer?"}
+  Q -- no --> A["stack.append(c) — it is an opener"]
+  A --> L
+  Q -- yes --> M{"stack non-empty and pairs[c] == stack[-1]?"}
+  M -- yes --> P["stack.pop() — matched"]
+  P --> L
+  M -- no --> F(["return False — mismatch"])
+```
+
 ## Solution
 
 ```py
@@ -52,3 +70,7 @@ class Solution:
 
         return True if not stack else False
 ```
+
+## Explanation
+
+<YouTube id="WTzjTskDFMg" title="Valid Parentheses - Stack - Leetcode 20 - Python" />