C++ code for Flattening a Binary Tree into a Doubly Linked List (multiple approaches)

# Aim

Objectives of the script:
1. Clear and concise code for flattening Binary Tree to Doubly Linked List
2. Decoding and understanding the process of traversing in a binary tree and creating a doubly-linked list simultaneously.

# Details

Features of the script:
1. Efficient and easy-to-understand code (fully working, along with the main function)
2. I will be covering multiple approaches mentioning varying time complexities (both iterative and recursive)
3. Step-by-step explanation of the solution will be provided
4. Can you assign me this issue under HacktoberFest 2021?

### Programming language

- [ ] C
- [x] C++
- [ ] Java
- [ ] Python

Start from the issue objectives and the C++ main function described in the details. Provide working iterative and recursive approaches for flattening a binary tree into a doubly linked list, with step-by-step explanation and time complexities; completion means the script runs and demonstrates the approaches.

Written by the indexing model from the issue text.