doc: Fix install target
doc: Fix install target

Maintain the directory structure instead of copying all files into one
directory.

Signed-off-by: Ola Jeppsson <793f970c52ded1276b9264c742f19d1888cbaf73@adapteva.com>

RafaelRMA/pal,eliteraspberries/pal,eliteraspberries/pal,twocs/pal,debug-de-su-ka/pal,8l/pal,twocs/pal,mateunho/pal,parallella/pal,mateunho/pal,olajep/pal,olajep/pal,olajep/pal,debug-de-su-ka/pal,8l/pal,aolofsson/pal,mateunho/pal,parallella/pal,twocs/pal,aolofsson/pal,parallella/pal,eliteraspberries/pal,RafaelRMA/pal,parallella/pal,twocs/pal,eliteraspberries/pal,8l/pal,aolofsson/pal,aolofsson/pal,debug-de-su-ka/pal,Adamszk/pal3,parallella/pal,RafaelRMA/pal,RafaelRMA/pal,olajep/pal,Adamszk/pal3,debug-de-su-ka/pal,eliteraspberries/pal,Adamszk/pal3,mateunho/pal,mateunho/pal,8l/pal,debug-de-su-ka/pal,Adamszk/pal3