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

apps/docs/content/(hash-table)/242-valid-anagram.mdx

@@ -25,6 +25,19 @@ What if the inputs contain Unicode characters? How would you adapt your solution
 - `1 <= s.length, t.length <= 5 * 10^4`
 - `s` and `t` consist of lowercase English letters.
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["isAnagram(s, t)"]) --> I["freq: count every char of s"]
+  I --> L{"more char c in t?"}
+  L -- no --> E(["return true"])
+  L -- yes --> Q{"freq[c] missing or 0?"}
+  Q -- yes --> R(["return false — t has a char s cannot cover"])
+  Q -- no --> D["freq[c] -= 1"]
+  D --> L
+```
+
 ## Solution
 
 ```js
@@ -46,3 +59,7 @@ var isAnagram = function(s, t) {
   return true;
 };
 ```
+
+## Explanation
+
+<YouTube id="9UtInBqnCgA" title="Valid Anagram - Leetcode 242 - Python" />