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

docs/solutions/(array)/303-range-sum-query-immutable.mdx

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+---
+title: '303. Range Sum Query - Immutable'
+description: Given an integer array nums, handle multiple queries of the following type
+sidebar:
+  label: 'Range Sum Query - Immutable'
+  badge: 'Easy'
+---
+
+<Badge variant="accent">Prefix Sum</Badge>
+
+### Example 1:
+- Input: `["NumArray", "sumRange", "sumRange", "sumRange"]` `[[[-2, 0, 3, -5, 2, -1]], [0, 2], [2, 5], [0, 5]]`
+- Output: `[null, 1, -1, -3]`
+- Explanation: `NumArray numArray = new NumArray([-2, 0, 3, -5, 2, -1]);` `numArray.sumRange(0, 2); // return (-2) + 0 + 3 = 1` `numArray.sumRange(2, 5); // return 3 + (-5) + 2 + (-1) = -1` `numArray.sumRange(0, 5); // return (-2) + 0 + 3 + (-5) + 2 + (-1) = -3`
+
+### Constraints:
+
+- `1 <= nums.length <= 10^4`
+- `-10^5 <= nums[i] <= 10^5`
+- `0 <= left <= right < nums.length`
+- At most `10^4` calls will be made to `sumRange`.
+
+## Solution
+
+```js
+/**
+ * @param {number[]} nums
+ */
+class NumArray {
+    constructor(nums) {
+      this.prefix = [0];
+      for (let n of nums){
+        this.prefix.push(this.prefix[this.prefix.length - 1] + n);
+      }
+    }
+    /**
+     * @param {number} left
+     * @param {number} right
+     * @return {number}
+     */
+    sumRange(left, right) {
+        return this.prefix[right + 1] - this.prefix[left];
+    }
+}
+
+
+/** 
+ * Your NumArray object will be instantiated and called as such:
+ * var obj = new NumArray(nums)
+ * var param_1 = obj.sumRange(left,right)
+ */
+```