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

apps/docs/content/(linked-list)/206-reverse-linked-list.mdx

@@ -29,6 +29,19 @@ A linked list can be reversed either iteratively or recursively. Could you imple
 - The number of nodes in the list is the range [0, 5000].
 - `-5000 <= Node.val <= 5000`
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["reverseList(head)"]) --> I["prev = None, curr = head"]
+  I --> W{"curr?"}
+  W -- no --> E(["return prev — the old tail is the new head"])
+  W -- yes --> A["next_ = curr.next — save it before overwriting"]
+  A --> B["curr.next = prev — flip the link"]
+  B --> C["prev = curr; curr = next_ — step forward"]
+  C --> W
+```
+
 ## Solution
 
 ```py
@@ -48,3 +61,7 @@ class Solution:
             curr = next_
         return prev
 ```
+
+## Explanation
+
+<YouTube id="G0_I-ZF0S38" title="Reverse Linked List - Iterative AND Recursive - Leetcode 206 - Python" />