Added SandcastlePath property to doc build so SHFB can find Sandcastle.
Added SandcastlePath property to doc build so SHFB can find Sandcastle.

zjmpds/Autofac,onovotny/Autofac,oconics/Autofac,laizhan/autofac,twsouthwick/Autofac,oconics/Autofac,lijianguang/Autofac,jruckert/Autofac,csantero/Autofac,liujunhua/Autofac,jango2015/Autofac,twsouthwick/Autofac,LexxFedoroff/Autofac,onovotny/Autofac,laizhan/autofac,pagebrooks/Autofac,LexxFedoroff/Autofac,neoe1984/https-github.com-autofac-Autofac,csantero/Autofac,jruckert/Autofac,zmaruo/Autofac,autofac/Autofac,neoe1984/https-github.com-autofac-Autofac,twsouthwick/Autofac,laizhan/autofac,laizhan/autofac,SeriaWei/Autofac,pagebrooks/Autofac,liujunhua/Autofac,SeriaWei/Autofac,twsouthwick/Autofac,twsouthwick/Autofac