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

docs/solutions/(array)/121-best-time-to-buy-and-sell-stock.mdx

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+---
+title: '121. Best Time to Buy and Sell Stock'
+description: You are given an array prices where prices[i] is the price of a given stock on the i^th day
+sidebar:
+  label: 'Best Time to Buy and Sell Stock'
+  badge: 'Easy'
+---
+
+<Badge variant="accent">Sliding Window</Badge>
+
+### Example 1:
+- Input: `prices = [7,1,5,3,6,4]`
+- Output: `5`
+- Explanation: Buy on day `2` (price = `1`) and sell on day `5` (price = `6`), profit = `6-1 = 5`. Note that buying on day `2` and selling on day `1` is not allowed because you must buy before you sell.
+
+### Example 2:
+- Input: `prices = [7,6,4,3,1]`
+- Output: `0`
+- Explanation: In this case, no transactions are done and the max profit = `0`.
+
+### Constraints:
+
+- `1 <= prices.length <= 10^5`
+- `0 <= prices[i] <= 10^4`
+
+## Solution
+
+```py
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        left = min(prices)
+        for right in range(len(prices)):
+            while curr > left:
+                curr -= prices[left]
+                left += 1
+            ans = max(ans, curr)
+        return ans
+```