docs(docs): add MDX page for Merge k Sorted Lists problem

apps/docs/content/(linked-list)/23-merge-k-sorted-lists.mdx

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+---
+title: '23. Merge k Sorted Lists'
+description: You are given an array of k linked-lists lists, each linked-list is sorted in ascending order
+sidebar:
+  label: 'Merge k Sorted Lists'
+  badge: 'Hard'
+---
+
+<Badge variant="accent">Heap / Priority Queue</Badge>
+
+### Example 1:
+- Input: `lists = [[1,4,5],[1,3,4],[2,6]]`
+- Output: `[1,1,2,3,4,4,5,6]`
+- Explanation: The linked-lists are: [ 1->4->5, 1->3->4, 2->6 ] merging them into one sorted linked list: 1->1->2->3->4->4->5->6
+
+### Example 2:
+- Input: `lists = []`
+- Output: `[]`
+
+### Example 3:
+- Input: `lists = [[]]`
+- Output: `[]`
+
+### Constraints:
+
+- `k == lists.length`
+- `0 <= k <= 10^4`
+- `0 <= lists[i].length <= 500`
+- `-10^4 <= lists[i][j] <= 10^4`
+- `lists[i]` is sorted in ascending order.
+- The sum of lists[i].length will not exceed 10^4.
+
+## Solution
+
+```py
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+import heapq
+
+
+class Solution:
+    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
+        heap = []
+
+        # K log K
+        for i, node in enumerate(lists):
+            if node:
+                heapq.heappush(heap, (node.val, i, node))
+
+        D = ListNode()
+        cur = D
+
+        # n log k
+        while heap:
+            val, i, node = heapq.heappop(heap)
+            cur.next = node
+            cur = node
+            node = node.next
+
+            if node:
+                heapq.heappush(heap, (node.val, i, node))
+
+        # Time: O(N log k)
+        # Space: O(n)
+        return D.next
+```