docs(docs): add Approach diagrams and Explanation sections to LeetCode problem documentation

apps/docs/content/(linked-list)/2-add-two-numbers.mdx

@@ -27,6 +27,20 @@ sidebar:
 - `0 <= Node.val <= 9`
 - It is guaranteed that the list represents a number that does not have leading zeros.
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["addTwoNumbers(l1, l2)"]) --> I["dummy = ListNode(), carry = 0, curr = dummy — dummy.next will be the head"]
+  I --> W{"l1 or l2 or carry?"}
+  W -- no --> E(["return dummy.next"])
+  W -- yes --> A["s = (l1.val if l1 else 0) + (l2.val if l2 else 0) + carry — a missing digit counts as 0"]
+  A --> D["carry, val = divmod(s, 10)"]
+  D --> N["curr.next = ListNode(val), curr = curr.next"]
+  N --> V["l1 = l1.next if l1 else None, l2 = l2.next if l2 else None"]
+  V --> W
+```
+
 ## Solution
 
 ```py
@@ -55,3 +69,7 @@ class Solution:
 
         return dummy.next
 ```
+
+## Explanation
+
+<YouTube id="wgFPrzTjm7s" title="Add Two Numbers - Leetcode 2 - Python" />