Add @System doxygen to perception systems

OK, there is only one right now, but it's a start.

perception/depth_image_to_point_cloud.h

@@ -17,8 +17,16 @@ namespace perception {
 /// Converts a depth image to a point cloud.
 /// Basically a system that wraps around ConvertDepthImageToPointCloud in
 /// RGBDCamera.
-/// The system has a single input port that takes a ImageDepth32F and a single
-/// output port that contains a PointCloud.
+/// The system has a single input port that takes a
+/// systems::sensors::ImageDepth32F and a single output port that contains a
+/// PointCloud.
+///
+/// @system{ DepthImageToPointCloud,
+///          @input_port{depth_image},
+///          @output_port{point_cloud}
+/// }
+///
+/// @ingroup perception_systems
 class DepthImageToPointCloud final : public systems::LeafSystem<double> {
  public:
   DRAKE_NO_COPY_NO_MOVE_NO_ASSIGN(DepthImageToPointCloud)

systems/systems.h

@@ -31,6 +31,7 @@
 ///   @defgroup automotive_systems Automotive Systems
 ///   @defgroup manipulation_systems Manipulation
 ///   @defgroup message_passing Message Passing
+///   @defgroup perception_systems Perception
 ///   @defgroup stochastic_systems Stochastic Systems
 ///   @defgroup visualization Visualization
 ///   @defgroup example_systems Examples
@@ -74,6 +75,12 @@
 // TODO(russt): Add pointers to / recommendations for connecting to ROS.
 // TODO(russt): Add ZMQ.
 
+/// @addtogroup perception_systems
+/// @{
+/// @brief Systems for dealing with perception data and/or wrapping basic
+/// perception algorithms.
+/// @}
+
 /// @addtogroup visualization
 /// @{
 /// @brief Systems for connecting to external visualization tools/GUIs.