Added C++ solution for 230 - Kth Smallest Element in a BST.

Change Summary

The purpose of this PR is to add coding solution in C++ for the LeetCode Kth Smallest Element in a BST problem.

If you haven't fulfilled the below requirements or even delete the entire checklist, your PR won't be reviewed and will be closed without notice.

[x] This Pull Request is all my own work. (You'll be blacklisted if you are caught for plagiarism.)
[x] I've read CONTRIBUTING.md
[x] I've applied LaTex for all variables, formulas and time / space complexity instead of using backticks
[x] I've started the app locally and verified all the content and all links (if applicable) are accessible correctly
[x] I've included Complexity Analysis (Time Complexity & Space Complexity).
[x] I've written my explanation well and it is easy to understand for beginners

[x] I've read and followed the Solution Template
[x] I've formatted my code well with K&R Coding style
[x] I've confirmed that comments are put above each line rather than writing on the same line.
[x] I've included a meaningful approach name for my solution. e.g. ## Approach 1: Two Pointers.