Document Point to Point Dumbbell Topology Helper
Add the documentation for Point to Point Dumbbell topology helper in ns-3-dev. This helper is implemented in the point to point layout module in ns-3-dev (ns-3-dev/src/point-to-point-layout/model/), but lacks documentation. First, you should create a new folder called ‘doc’ inside ns-3-dev/src/point-to-point-layout/ and then create a new file called ‘point-to-point-dumbbell.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.