Implement DFS in C++

## 🔧 Implement: Depth-First Search (DFS) in C++

File path:
```
algo-and-data-structures/graphs/dfs.cpp
```

### Requirements:
- Implement DFS using recursion and adjacency list.
- Print the order of traversal.

✅ Test:
```bash
g++ dfs.cpp -o dfs && ./dfs
```
