Document multi-uav sim with sdf files

[lamping7]

Need to add dependency: xmlstarlet and a brief description due to PX4/PX4-Autopilot#12306

[hamishwillee]

That sounds cool - do you plan on doing this doc update?

Is this a new doc or an extension of the old one? https://dev.px4.io/master/en/simulation/multi-vehicle-simulation.html structure?

Note that I don't fully understand the solution and how it intersects with the old solution (ie what is the same and what is is different - which is why I don't know if this is an addition to the old doc or a completely alternative option).
(I THINK that the essential difference is that instead of hard coding the port for each model in the xacro you have a single definition and use this xmlstarlet thing to create multiple mavlink_tcp_port thingies. I think this is a solution if the vehicle is the same in each case, but won't work if you you need multiple types of vehicles - presumably because you're copying from a single model. I think that most of the other things to do the setup are the same - Ie you will still need rcS startup files for each vehicle.
But all this is a guess.

What I do know is that we will need to add the limitations from the linked firmware issue and perhaps expand a little on what they mean to end users. If this is a new doc, it should compare and contrast to the other solution.

It would also be good to test on Ubuntu 18.04 and ROS Melodic as this is our new default target for CI.