Provide SDL binaries for the C++ example or add further instructions

[KonanM]

The folder
https://github.com/microsoft/WinUI-3-Demos/tree/master/src/ContosoAirlinePOS/ContosoAirlinePOSCpp/ContosoSeatBooking/SDL2-devel-2.0.12-VC/SDL2-2.0.12
only contains the header files, but not the (x64) binaries (for SDL + SDL image) to be able to build the C++ example.
So I would propose to either add the binaries/ use the SDL nuget package for them or add build instructions that those binaries have to be downloaded and put there.