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

apps/docs/content/(hash-table)/141-linked-list-cycle.mdx

@@ -33,6 +33,20 @@ Can you solve it using O(1) (i.e. constant) memory?
 - `-10^5 <= Node.val <= 10^5`
 - pos is -1 or a valid index in the linked-list.
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["hasCycle(head)"]) --> I["fast = slow = head"]
+  I --> W{"fast and fast.next?"}
+  W -- no --> E(["return False — ran off the end, no cycle"])
+  W -- yes --> A["fast = fast.next.next — two steps"]
+  A --> B["slow = slow.next — one step"]
+  B --> Q{"fast is slow?"}
+  Q -- yes --> R(["return True — the gap closed, so it loops"])
+  Q -- no --> W
+```
+
 ## Solution
 
 ```py
@@ -54,3 +68,7 @@ class Solution:
                 return True
         return False
 ```
+
+## Explanation
+
+<YouTube id="gBTe7lFR3vc" title="Linked List Cycle - Floyd's Tortoise and Hare - Leetcode 141 - Python" />