docs(docs): restructure documentation directories and remove placeholder study decks

docs/solutions/03-3sum.mdx

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+---
+title: '15. 3Sum'
+description: Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]] such that i != j, i != k, and j != k, and nums[i] + nums[j] + nums[k] == 0
+sidebar:
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Two Pointers</Badge>
+
+::::warning
+Notice that the solution set must not contain duplicate triplets.
+::::
+
+### Example 1:
+- Input: `nums = [-1,0,1,2,-1,-4]`
+- Output: `[[-1,-1,2],[-1,0,1]]`
+- Explanation: `nums[0] + nums[1] + nums[2] = (-1) + 0 + 1 = 0`. `nums[1] + nums[2] + nums[4] = 0 + 1 + (-1) = 0`. `nums[0] + nums[3] + nums[4] = (-1) + 2 + (-1) = 0`. The distinct triplets are `[-1,0,1]` and `[-1,-1,2]`. Notice that the order of the output and the order of the triplets does not matter.
+
+### Example 2:
+- Input: `nums = [0,1,1]`
+- Output: `[]`
+- Explanation: The only possible triplet does not sum up to `0`.
+
+### Example 3:
+- Input: `nums = [0,0,0]`
+- Output: `[[0,0,0]]`
+- Explanation: The only possible triplet sums up to `0`.
+
+### Constraints:
+
+- `3 <= nums.length <= 3000`
+- `-10^5 <= nums[i] <= 10^5`
+
+## Solution
+
+```js
+/**
+ * @param {number[]} nums
+ * @return {number[][]}
+ */
+var threeSum = function(nums) {
+    
+};
+```