Add documentation on how to handle multiple inputs/gamepads

I've scoured the internet on written documentation on how to handle multiple gamepads/controllers and I couldn't find anything official on it. Besides random reddit comments.

As far as I'm aware there is only [this](https://docs.godotengine.org/en/stable/tutorials/inputs/controllers_gamepads_joysticks.html) talking about input handling and it never mentions anything about handling inputs for more than one device.
I would be nice to have official docs on the ways one could implement this for a local multiplayer game (using multiple input maps and handling device ids to avoid maintaining multiple input maps???).

Start with the linked controllers and gamepads documentation page and review the existing input-handling guidance. Document practical approaches for local multiplayer with multiple devices, including input maps and device IDs, and consider the trade-offs; it is done when official guidance explains how to handle more than one gamepad.

Written by the indexing model from the issue text.