what is the difference of the folder t_reconstruction_system and reconstruction_system ?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](https://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](https://www.open3d.org/docs/release/) and the [latest documentation](https://www.open3d.org/docs/latest/) (for `main` branch).

### My Question

Hi, I am learning the reconstruction system and in the official tutorial [Quick start](https://www.open3d.org/docs/release/tutorial/reconstruction_system/system_overview.html) , the code asks me to direct to folder **reconstruction_system** and then execute the **dense_slam_gui.py**:
`cd examples/python/reconstruction_system/ `
`python dense_slam_gui.py --help`

But from the latest main branch I could only find dense_slam_gui.py in folder **t_reconstruction_system**.

Then the following tutorial asks me to execute **run_system.py** which exists **in both** t_reconstruction_system folder as well as reconstruction_system folder.

My question is, what is the difference of the folder **t_reconstruction_system** and **reconstruction_system** ? and can I just use the run_system.py in t_reconstruction_system folder?