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

apps/docs/content/(array)/49-group-anagrams.mdx

@@ -27,6 +27,21 @@ sidebar:
 - `0 <= strs[i].length <= 100`
 - `strs[i]` consists of lowercase English letters.
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["groupAnagrams(strs)"]) --> K["sorted[i] = letters of strs[i], sorted — the anagram key"]
+  K --> I["anagrams = {}"]
+  I --> L{"more i in 0..n-1?"}
+  L -- no --> E(["return Object.values(anagrams)"])
+  L -- yes --> Q{"anagrams has sorted[i]?"}
+  Q -- no --> N["anagrams[sorted[i]] = [strs[i]]"]
+  Q -- yes --> P["anagrams[sorted[i]].push(strs[i])"]
+  N --> L
+  P --> L
+```
+
 ## Solution
 
 ```js
@@ -49,3 +64,7 @@ var groupAnagrams = function(strs) {
   return Object.values(anagrams);
 };
 ```
+
+## Explanation
+
+<YouTube id="vzdNOK2oB2E" title="Group Anagrams - Categorize Strings by Count - Leetcode 49" />