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

apps/docs/content/(two-pointers)/344-reverse-string.mdx

@@ -25,6 +25,18 @@ You must do this by modifying the input array in-place with O(1) extra memory.
 - `1 <= s.length <= 10^5`
 - `s[i]` is a printable ascii character.
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["reverseString(s)"]) --> I["i = 0, j = s.length - 1"]
+  I --> W{"i < j?"}
+  W -- no --> E(["done — s was reversed in place, nothing returned"])
+  W -- yes --> A["swap s[i] and s[j]"]
+  A --> B["i++; j--"]
+  B --> W
+```
+
 ## Solution
 
 ```js
@@ -43,3 +55,7 @@ var reverseString = function(s) {
     }   
 };
 ```
+
+## Explanation
+
+<YouTube id="_d0T_2Lk2qA" title="Reverse String - 3 Ways - Leetcode 344 - Python" />