Populate classId and confidence level for object detection in Java simulation (#2372)

## Description

compute confidence level based on target area in total image size and
populate classId and confidence level in Java (while building the
PhotonTrackedTarget)

## Changes
- Add new VisionTargetSim constructor for object detection
- If class ID specified but confidence = -1, estimate based on total
area

---------

Co-authored-by: Matt Morley <matthew.morley.ca@gmail.com>

docs/source/docs/simulation/simulation-java.md

@@ -95,6 +95,27 @@ These `TargetModel` are paired with a target pose to create a `VisionTargetSim`.
 The pose of a `VisionTargetSim` object can be updated to simulate moving targets. Note, however, that this will break latency simulation for that target.
 :::
 
+To use simulated object detection, you must provide an objDetClassId (zero-indexed class ID) and confidence value. When you set objDetConf to -1, the simulation computes confidence based on the area of the target in the camera's field of view. To simulate a object detection model with one class (fuel, index 0) and specify confidence, you'd write:
+
+```{eval-rst}
+.. tab-set-code::
+
+   .. code-block:: java
+
+      // arbitrary position on field
+      final var targetPose = new Pose3d(new Translation3d(2, 0, 0), new Rotation3d());
+      // Class id, zero-indexed
+      final int classId = 0;
+      // Confidence, between 0 and 1.
+      final float conf = 0.67f;
+      // 6 inch diameter ball
+      final TargetModel ballModel = new TargetModel(Units.inchesToMeters(6));
+      final var ballTargetSim = new VisionTargetSim(targetPose, ballModel, classId, conf);
+
+      // Add this vision target to the vision system simulation to make it visible
+      visionSim.addVisionTargets(visionTarget);
+```
+
 For convenience, an `AprilTagFieldLayout` can also be added to automatically create a target for each of its AprilTags.
 
 ```{eval-rst}