enhancement & docs: rendering method set to mobile by default

[abmarnie]

In project.godot,

[rendering]

renderer/rendering_method="mobile"

but some people might not be making mobile games, so it would be good to remind them somewhere to change this in project settings (or maybe even let them choose which rendering_method to use when they do dotnet new chickengame, that way they don't need to go into the editor, change this, and then restart the editor for new projects).