docs: add sidebar labels and reorganize solution docs into category folders

docs/solutions/(array)/42-trapping-rain-water.mdx

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+---
+title: '42. Trapping Rain Water'
+description: Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining
+sidebar:
+  label: 'Trapping Rain Water'
+  badge: 'Hard'
+---
+
+<Badge variant="accent">Two Pointers</Badge>
+
+### Example 1:
+- Input: `height = [0,1,0,2,1,0,1,3,2,1,2,1]`
+- Output: `6`
+- Explanation: The above elevation map (black section) is represented by array `[0,1,0,2,1,0,1,3,2,1,2,1]`. In this case, `6` units of rain water (blue section) are being trapped.
+
+### Example 2:
+- Input: `height = [4,2,0,3,2,5]`
+- Output: `9`
+
+### Constraints:
+
+- `n == height.length`
+- `1 <= n <= 2 * 10^4`
+- `0 <= height[i] <= 10^5`
+
+## Solution
+
+```js
+/**
+ * @param {number[]} height
+ * @return {number}
+ */
+var trap = function(height) {
+    
+};
+```