docs(welcome): restructure documentation directories and add new guides section

docs/(welcome)/array/347.mdx

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+---
+title: LC-347
+description: Top K Frequent Elements
+---
+
+## Examples
+
+| Input                              | Output   |
+| ---------------------------------- | -------- |
+| `nums = [1,1,1,2,2,3], k = 2`      | `[1,2]`  |
+| `nums = [1], k = 1`                | `[1]`    |
+| `nums = [1,2,1,2,1,2,3,1,3,2], k = 2` | `[1,2]` |
+
+## Hints
+
+<Accordion type="single">
+  <AccordionItem title="Constraints">
+    Constraints:
+     • `1 <= nums.length <= 10^5`
+     • `-10^4 <= nums[i] <= 10^4`
+     • `k` is in the range `[1, the number of unique elements in the array]`
+     • It is guaranteed that the answer is unique.
+  </AccordionItem>
+  <AccordionItem title="Problem Statement">
+    Given an integer array nums and an integer `k`, return the `k` most frequent elements.
+    You may return the answer in any order.
+
+    Follow up: Your algorithm's time complexity must be better than `O(n log n)`, where n is the array's size.
+  </AccordionItem>
+</Accordion>
+
+## Solution
+
+```js
+/**
+ * @param {number[]} nums
+ * @param {number} k
+ * @return {number[]}
+ */
+
+var topKFrequent = function (nums, k) {
+  // Default for Hammer
+  const freq = {};
+  for (let n of nums) freq[n] = (freq[n] || 0) + 1;
+
+  // Problem Specific
+  // Sort unique values by descending frequency, take the first k
+  return Object.keys(freq)
+    .map(Number)
+    .sort((a, b) => freq[b] - freq[a])
+    .slice(0, k);
+};
+```