Continuous-wave Doppler-free two-photon spectroscopy of the $\mathsf{4d^{10}5s}$$\mathsf{^{2}}$S $\mathsf{_{1/2}\rightarrow 4d^{9}5s^{2}}$$\mathsf{^{2}}$D $\mathsf{_{3/2}}$ transition in atomic silver