阅读下列一段文字,回答问题.
在平面直角坐标内有两点$M(x_{1}$,$y_{1})$,$N(x_{2}$,$y_{2})$,连接$M$,$N$,则线段$MN$的中点坐标为$(\frac{{x}_{1}+{x}_{2}}{2},\frac{{y}_{1}+{y}_{2}}{2})$,例如,点$M\left(-1,2\right)$,$N\left(3,4\right)$,则线段$MN$的中点坐标为$(\frac{-1+3}{2},\frac{2+4}{2})$,即$\left(1,3\right)$.
$(1)$已知点$P\left(-2,6\right)$,$Q\left(0,2\right)$,则线段$PQ$的中点坐标为______.
$(2)$如图,直线$y=2x+4$与坐标轴分别交于$A$,$B$两点,点$C$坐标为$\left(5,0\right)$,线段$CD$是$\triangle ABC$的中线,求$CD$的长