Add documentation for Auto Rate Fallback model
Add the documentation for Auto Rate Fallback in ns-3-dev. This Rate adaptation algorithm is implemented in the wifi module in ns-3-dev, but lacks documentation. First, you should go to the folder called ‘doc’ inside ns-3-dev/src/wifi/ and then create a new file called ‘arf-wifi-manager.rst’ to start the documentation. As an example, you can refer to the documentation of point to point network device (src/point-to-point/doc/point-to-point.rst). Submit the new .rst file for evaluation.