Synthesis and characterisation of Cu0.5Mg0.5Fe2O4 ${\text{Cu}}_{\mathbf{0.5}}{\text{Mg}}_{\mathbf{0.5}}{\text{Fe}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{4}}$ nanoparticles doped with cadmium by co‐precipitation method for acetonitrile, acetone, and ethanol gas detection with deep learning‐based methods