docs(docs): add documentation for Binary Tree Right Side View problem

apps/docs/content/(tree)/199-binary-tree-right-side-view.mdx

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+---
+title: '199. Binary Tree Right Side View'
+description: Given the root of a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom
+sidebar:
+  label: 'Binary Tree Right Side View'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Tree BFS</Badge>
+
+### Example 1:
+- Input: `root = [1,2,3,null,5,null,4]`
+- Output: `[1,3,4]`
+
+### Example 2:
+- Input: `root = [1,2,3,4,null,null,null,5]`
+- Output: `[1,3,4,5]`
+
+### Example 3:
+- Input: `root = [1,null,3]`
+- Output: `[1,3]`
+
+### Example 4:
+- Input: `root = []`
+- Output: `[]`
+
+### Constraints:
+
+- The number of nodes in the tree is in the range [0, 100].
+- `-100 <= Node.val <= 100`
+
+## Solution
+
+```py
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+from collections import deque
+
+
+class Solution:
+    def rightSideView(self, root: Optional[TreeNode]) -> List[int]:
+        res = []
+        q = deque([root])
+
+        while q:
+            rightSide = None
+            qLen = len(q)
+
+            for i in range(qLen):
+                node = q.popleft()
+                if node:
+                    rightSide = node
+                    q.append(node.left)
+                    q.append(node.right)
+
+            if rightSide:
+                res.append(rightSide.val)
+
+        return res
+```